gcc.c-torture/execute/builtins/fputs.c: fputs_unlocked prototype

gcc.c-torture/execute/builtins/fputs.c: fputs_unlocked prototype

Current glibc headers only declare fputs_unlocked for _GNU_SOURCE,
so define it to obtain an official prototype.

Add a fallback prototype declaration for other systems that do not
have fputs_unlocked.  This seems to the most straightforward approach
to avoid an implicit function declaration, without reducing test
coverage and introducing ongoing maintenance requirements (e.g.,
FreeBSD added fputs_unlocked support fairly recently).

gcc/testsuite/

	* gcc.c-torture/execute/builtins/fputs.c (_GNU_SOURCE):
	Define.
	(fputs_unlocked): Declare.