Always run testsuite with same GDCFLAGS as used in build.

libphobos/ChangeLog:

	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Remove GDCFLAGSX.
	* libdruntime/Makefile.in: Regenerate.
	* src/Makefile.in: Regenerate.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/testsuite_flags.in: Use GDCFLAGS in --gdcflags.
	* testsuite/libphobos.thread/fiber_guard_page.d: Test using -O0.