runtime: set _XOPEN_SORUCE_EXTENDED to 1 for aix

aix requires that _XOPEN_SOURCE_EXTENDED is set to a number, not simply
defined.

For #62440.

Change-Id: Iee221d558b5ad5b8dcb874d4d9fdf94593f7d0a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/527797
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>
Run-TryBot: Michael Pratt <[email protected]>

Author: prattmic

src/runtime/testdata/testprogcgo/stackswitch.c

@@ -7,7 +7,7 @@
 // Required for darwin ucontext.
 #define _XOPEN_SOURCE
 // Required for netbsd stack_t if _XOPEN_SOURCE is set.
-#define _XOPEN_SOURCE_EXTENDED
+#define _XOPEN_SOURCE_EXTENDED	1
 #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
 
 #include <assert.h>