Fix bug-strspn1.c, bug-strpbrk1.c build with GCC mainline.

Fix bug-strspn1.c, bug-strpbrk1.c build with GCC mainline.

Building the testsuite with GCC mainline fails with:

bug-strspn1.c: In function 'main':
bug-strspn1.c:14:3: error: right-hand operand of comma expression has no effect [-Werror=unused-value]
   strspn (b++, "");
   ^~~~~~~~~~~~~~~~

and a similar error for bug-strpbrk1.c.  I'm not sure what GCC change
introduced this, and the wording of the message is a bit off (in the
source it's not a comma expression, that must reflect GCC's IR).  But
the warning is correct (strspn is a pure function, the call is
useless, and if there wasn't an argument with a side effect much older
GCC would have warned); the point of the test is to verify that the
side effect in an argument still occurs for this useless call that can
otherwise be optimized to an (unused) constant (testing for a bug
there once was in an old strspn macro).  This patch duly arranges for
the warning to be disabled for this code.

Tested with build-many-glibcs.py for aarch64-linux-gnu.

	* string/bug-strpbrk1.c: Include <libc-diag.h>.
	(main): Disable -Wunused-value around call to strpbrk.
	* string/bug-strspn1.c: Include <libc-diag.h>.
	(main): Disable -Wunused-value around call to strspn.