work around gdb issues recognizing sigreturn trampoline on x86_64

gdb can only backtrace/unwind across signal handlers if it recognizes
the sa_restorer trampoline. for x86_64, gdb first attempts to
determine the symbol name for the function in which the program
counter resides and match it against "__restore_rt". if no name can be
found (e.g. in the case of a stripped binary), the exact instruction
sequence is matched instead.

when matching the function name, however, gdb's unwind code wrongly
considers the interval [sym,sym+size] rather than [sym,sym+size).
thus, if __restore_rt begins immediately after another function, gdb
wrongly identifies pc as lying within the previous adjacent function.
this patch adds a nop before __restore_rt to preclude that
possibility. it also removes the symbol name __restore and replaces it
with a macro since the stability of whether gdb identifies the
function as __restore_rt or __restore is not clear.

for the no-symbols case, the instruction sequence is changed to use
%rax rather than %eax to match what gdb expects.

based on patch by Szabolcs Nagy, with extended description and
corresponding x32 changes added.

arch/x32/ksigaction.h

@@ -0,0 +1,9 @@
+struct k_sigaction {
+	void (*handler)(int);
+	unsigned long flags;
+	void (*restorer)(void);
+	unsigned mask[2];
+};
+
+void __restore_rt();
+#define __restore __restore_rt

arch/x86_64/ksigaction.h

@@ -0,0 +1,9 @@
+struct k_sigaction {
+	void (*handler)(int);
+	unsigned long flags;
+	void (*restorer)(void);
+	unsigned mask[2];
+};
+
+void __restore_rt();
+#define __restore __restore_rt

src/signal/x32/restore.s

@@ -1,8 +1,7 @@
+	nop
 .global __restore_rt
-.global __restore
 .type __restore_rt,@function
-.type __restore,@function
 __restore_rt:
-__restore:
-	movl $0x40000201, %eax /* SYS_rt_sigreturn */
+	mov $0x40000201, %rax /* SYS_rt_sigreturn */
 	syscall
+.size __restore_rt,.-__restore_rt

src/signal/x86_64/restore.s

@@ -1,8 +1,7 @@
+	nop
 .global __restore_rt
-.global __restore
 .type __restore_rt,@function
-.type __restore,@function
 __restore_rt:
-__restore:
-	movl $15, %eax
+	mov $15, %rax
 	syscall
+.size __restore_rt,.-__restore_rt