vxworks: Fix GCC selftests for *-wrs-vxworks7-* targets

vxworks: Fix GCC selftests for *-wrs-vxworks7-* targets

Currently when building a cross-compiler targeting arm-wrs-vxworks7, the
self-tests fail unless the VSB_DIR environment variable is set.

This prevents attempts at designating the location of runtime header
files, libraries or startfiles, which would fail on unset environment
variables and aren't needed for such tests.

gcc/ChangeLog:

	* config/vxworks.h (VXWORKS_ADDITIONAL_CPP_SPEC): Don't include
	VxWorks header files if -fself-test is used.
	(STARTFILE_PREFIX_SPEC): Avoid using VSB_DIR if -fself-test is used.