runtime: set _XOPEN_SOURCE_EXTENDED for netbsd

If _XOPEN_SOURCE is defined, then netbsd also requires
_XOPEN_SOURCE_EXTENDED to define stack_t.

For #62440.

Change-Id: Ib05658c3ca7fae1f6b051566e713ce7bc7c037a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/527775
Run-TryBot: Michael Pratt <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: prattmic

src/runtime/testdata/testprogcgo/stackswitch.c

@@ -6,6 +6,8 @@
 
 // Required for darwin ucontext.
 #define _XOPEN_SOURCE
+// Required for netbsd stack_t if _XOPEN_SOURCE is set.
+#define _XOPEN_SOURCE_EXTENDED
 #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
 
 #include <assert.h>