testsuite: Prevent spellcheck-inttypes failures on AIX.

testsuite: Prevent spellcheck-inttypes failures on AIX.

AIX stdio.h implicitly includes sys/types.h, which implicitly includes
inttypes.h.  With a recent AIX header fixincludes change to unilaterally
define STDC Macros, the GCC testsuite uses of inttypes now fails.

This patch explicitly defines the _STD_TYPES_T macro when the test is
run on AIX so that the inttypes.h header behaves as the testcase requires.

gcc/testsuite/ChangeLog:

2020-09-29  David Edelsohn  <dje.gcc@gmail.com>

	* g++.dg/spellcheck-inttypes.C: Define _STD_TYPES_T on AIX.
	* gcc.dg/spellcheck-inttypes.c: Same.