fix MINSIGSTKSZ values for archs with large signal contexts

the previous values (2k min and 8k default) were too small for some
archs. aarch64 reserves 4k in the signal context for future extensions
and requires about 4.5k total, and powerpc reportedly uses over 2k.
the new minimums are chosen to fit the saved context and also allow a
minimal signal handler to run.

since the default (SIGSTKSZ) has always been 6k larger than the
minimum, it is also increased to maintain the 6k usable by the signal
handler. this happens to be able to store one pathname buffer and
should be sufficient for calling any function in libc that doesn't
involve conversion between floating point and decimal representations.

x86 (both 32-bit and 64-bit variants) may also need a larger minimum
(around 2.5k) in the future to support avx-512, but the values on
these archs are left alone for now pending further analysis.

the value for PTHREAD_STACK_MIN is not increased to match MINSIGSTKSZ
at this time. this is so as not to preclude applications from using
extremely small thread stacks when they know they will not be handling
signals. unfortunately cancellation and multi-threaded set*id() use
signals as an implementation detail and therefore require a stack
large enough for a signal context, so applications which use extremely
small thread stacks may still need to avoid using these features.

arch/aarch64/bits/signal.h

@@ -1,6 +1,11 @@
 #if defined(_POSIX_SOURCE) || defined(_POSIX_C_SOURCE) \
  || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 
+#if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+#define MINSIGSTKSZ 6144
+#define SIGSTKSZ 12288
+#endif
+
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 typedef unsigned long greg_t;
 typedef unsigned long gregset_t[34];

arch/arm/bits/signal.h

@@ -1,6 +1,11 @@
 #if defined(_POSIX_SOURCE) || defined(_POSIX_C_SOURCE) \
  || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 
+#if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+#define MINSIGSTKSZ 2048
+#define SIGSTKSZ 8192
+#endif
+
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 typedef int greg_t, gregset_t[18];
 typedef struct sigcontext

arch/i386/bits/signal.h

@@ -1,6 +1,11 @@
 #if defined(_POSIX_SOURCE) || defined(_POSIX_C_SOURCE) \
  || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 
+#if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+#define MINSIGSTKSZ 2048
+#define SIGSTKSZ 8192
+#endif
+
 #ifdef _GNU_SOURCE
 #define REG_GS          0
 #define REG_FS          1

arch/microblaze/bits/signal.h

@@ -1,6 +1,11 @@
 #if defined(_POSIX_SOURCE) || defined(_POSIX_C_SOURCE) \
  || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 
+#if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+#define MINSIGSTKSZ 2048
+#define SIGSTKSZ 8192
+#endif
+
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 typedef unsigned long greg_t, gregset_t[38];
 typedef struct sigcontext

arch/mips/bits/signal.h

@@ -1,6 +1,11 @@
 #if defined(_POSIX_SOURCE) || defined(_POSIX_C_SOURCE) \
  || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 
+#if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+#define MINSIGSTKSZ 2048
+#define SIGSTKSZ 8192
+#endif
+
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 typedef unsigned long long greg_t, gregset_t[32];
 typedef struct {

arch/or1k/bits/signal.h

@@ -1,6 +1,11 @@
 #if defined(_POSIX_SOURCE) || defined(_POSIX_C_SOURCE) \
  || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 
+#if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+#define MINSIGSTKSZ 2048
+#define SIGSTKSZ 8192
+#endif
+
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 typedef unsigned long greg_t, gregset_t[34];
 typedef struct sigcontext {

arch/powerpc/bits/signal.h

@@ -1,6 +1,11 @@
 #if defined(_POSIX_SOURCE) || defined(_POSIX_C_SOURCE) \
  || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 
+#if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+#define MINSIGSTKSZ 4096
+#define SIGSTKSZ 10240
+#endif
+
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 
 typedef unsigned long greg_t, gregset_t[48];

arch/sh/bits/signal.h

@@ -1,6 +1,11 @@
 #if defined(_POSIX_SOURCE) || defined(_POSIX_C_SOURCE) \
  || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 
+#if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+#define MINSIGSTKSZ 2048
+#define SIGSTKSZ 8192
+#endif
+
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 typedef int greg_t, gregset_t[16];
 typedef int freg_t, fpregset_t[16];

arch/x32/bits/signal.h

@@ -1,6 +1,11 @@
 #if defined(_POSIX_SOURCE) || defined(_POSIX_C_SOURCE) \
  || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 
+#if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+#define MINSIGSTKSZ 2048
+#define SIGSTKSZ 8192
+#endif
+
 #ifdef _GNU_SOURCE
 #define REG_R8          0
 #define REG_R9          1

arch/x86_64/bits/signal.h

@@ -1,6 +1,11 @@
 #if defined(_POSIX_SOURCE) || defined(_POSIX_C_SOURCE) \
  || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 
+#if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+#define MINSIGSTKSZ 2048
+#define SIGSTKSZ 8192
+#endif
+
 #ifdef _GNU_SOURCE
 #define REG_R8          0
 #define REG_R9          1

include/signal.h

@@ -222,8 +222,6 @@ void (*sigset(int, void (*)(int)))(int);
 #define POLL_HUP 6
 #define SS_ONSTACK    1
 #define SS_DISABLE    2
-#define MINSIGSTKSZ 2048
-#define SIGSTKSZ 8192
 #endif
 
 #if defined(_BSD_SOURCE) || defined(_GNU_SOURCE)