Support expected failures in .test-result files.

This patch, an updated version of
<https://sourceware.org/ml/libc-alpha/2014-01/msg00195.html>, makes it
possible for .test-result files for individual tests to contain XPASS
and XFAIL rather than PASS and FAIL in cases where failure is
expected.  This replaces the marking of two individual tests with "-"
to cause them to be expected at makefile level to fail;
evaluate-test.sh will ensure it exits with status 0 for an expected
failure.

Tested x86_64.

	* scripts/evaluate-test.sh: Take new argument indicating whether
	failure is expected.
	* Makeconfig (evaluate-test): Pass argument to evaluate-test.sh
	indicating whether failure is expected.
	* conform/Makefile (test-xfail-run-conformtest): New variable.
	($(objpfx)run-conformtest.out): Don't expect to fail at makefile
	level.
	* posix/Makefile (test-xfail-annexc): New variable.
	($(objpfx)annexc.out): Don't expect to fail at makefile level.