introduce target fileio and require it in tests that use tmpnam

Some target C libraries that aren't recognized as freestanding don't
have filesystem support, so calling tmpnam, fopen/open and
remove/unlink fails to link.

This patch introduces a fileio effective target to the testsuite, and
requires it in the tests that call tmpnam.


for  gcc/testsuite/ChangeLog

	* lib/target-supports.exp (check_effective_target_fileio): New.
	* gcc.c-torture/execute/fprintf-2.c: Require it.
	* gcc.c-torture/execute/printf-2.c: Likewise.
	* gcc.c-torture/execute/user-printf.c: Likewise.