run sysconf-requiring test on systems that support it

run sysconf-requiring test on systems that support it

Some gcc.target/i386 tests requires the mmap feature, but that's not
enough for the test to be able to call sysconf.

This patch introduces a sysconf feature, analogous to mmap, and adds
it to tests in gcc.target/i386 that call sysconf.

There are other tests within gcc.dg and g++.dg that call sysconf, but
I haven't added the tag to them, because they already cover it with
target triplets.  I was a little nervous about dropping the triplets,
and saw how they implied sysconf, so I left those alone.

for  gcc/ChangeLog

	* doc/sourcebuild.texi (sysconf): New effective target.

for  gcc/testsuite/ChangeLog

	* lib/target-supports.exp (check_effective_target_sysconf): New.
	* gcc.target/i386/pr95443-1.c: Require it.
	* gcc.target/i386/pr95443-2.c: Likewise.
	* gcc.target/i386/sse2-mmx-maskmovq.c: Likewise.
	* gcc.target/i386/strncmp-1.c: Likewise.