Ignore -Wmaybe-uninitialized in stdlib/bug-getcontext.c.

Doing all-ABIs compile testing produces a compiler warning in
stdlib/bug-getcontext.c on nios2 and tilepro (with GCC 5 branch):

bug-getcontext.c: In function 'do_test':
bug-getcontext.c:53:6: error: 'except_mask' may be used uninitialized in this function [-Werror=maybe-uninitialized]
   if (mask != except_mask)
      ^

This warning appears nonsensical; except_mask is initialized where
it's declared.  I think what must be happening here is that the
compiler is confused by the returns-twice nature of getcontext: if
there were a call to setcontext, local variables could indeed have
lost their values on the second return from getcontext.  This patch
duly uses the DIAG_* macros to disable the warning here.

Tested for nios2 and tilepro (compilation only; after this patch all
the tests compile, though there are other failures) and x86_64 (full
testsuite run).

	* stdlib/bug-getcontext.c: Include <libc-internal.h>.
	(do_test): Disable -Wmaybe-uninitialized around uses of
	except_mask.