[libstdc++] [testsuite] no libatomic for vxworks

[libstdc++] [testsuite] no libatomic for vxworks

libatomic hasn't been ported to vxworks.  Most of the stdatomic.h and
<atomic> underlying requirements are provided by builtins and libgcc,
and the vxworks libc already provides remaining __atomic symbols, so
porting libatomic doesn't seem to make sense.

However, some of the target arch-only tests in
add_options_for_libatomic cover vxworks targets, so we end up
attempting to link libatomic in, even though it's not there.
Preempt those too-broad tests.

Co-Authored-By: Marc Poulhiès <poulhies@adacore.com>
for  libstdc++-v3/ChangeLog

	* testsuite/lib/dg-options.exp (add_options_for_libatomic):
	None for *-*-vxworks*.