fix build regression on archs with variable page size

commit 31fb174dd2 used
DEFAULT_GUARD_SIZE from pthread_impl.h in a static initializer,
breaking build on archs where its definition, PAGE_SIZE, is not a
constant. instead, just define DEFAULT_GUARD_SIZE as 4096, the minimal
page size on any arch we support. pthread_create rounds up to whole
pages anyway, so defining it to 1 would also work, but a moderately
meaningful value is nicer to programs that use
pthread_attr_getguardsize on default-initialized attribute objects.

src/internal/pthread_impl.h

@@ -143,7 +143,7 @@ void __block_app_sigs(void *);
 void __restore_sigs(void *);
 
 #define DEFAULT_STACK_SIZE 81920
-#define DEFAULT_GUARD_SIZE PAGE_SIZE
+#define DEFAULT_GUARD_SIZE 4096
 
 #define __ATTRP_C11_THREAD ((void*)(uintptr_t)-1)