ldso: fix non-functional fix to early dynamic PAGE_SIZE access

commit f47a8cdd250d9163fcfb39bf4e9d813957c0b187 introduced an
alternate mechanism for access to runtime page size for compatibility
with early stages of dynamic linking, but because pthread_impl.h
indirectly includes libc.h, the condition #ifndef PAGE_SIZE was never
satisfied.

rather than depend on order of inclusion, use the (baseline POSIX)
macro PAGESIZE, not the (XSI) macro PAGE_SIZE, to determine whether
page size is dynamic. our internal libc.h only provides a dynamic
definition for PAGE_SIZE, not for PAGESIZE.

ldso/dynlink.c

@@ -21,15 +21,17 @@
 #include <sys/membarrier.h>
 #include "pthread_impl.h"
 #include "fork_impl.h"
+#include "libc.h"
 #include "dynlink.h"
 
 static size_t ldso_page_size;
-#ifndef PAGE_SIZE
+/* libc.h may have defined a macro for dynamic PAGE_SIZE already, but
+ * PAGESIZE is only defined if it's constant for the arch. */
+#ifndef PAGESIZE
+#undef PAGE_SIZE
 #define PAGE_SIZE ldso_page_size
 #endif
 
-#include "libc.h"
-
 #define malloc __libc_malloc
 #define calloc __libc_calloc
 #define realloc __libc_realloc