libstdc++: Use libatomic for tests on all 32-bit powerpc targets

libstdc++: Use libatomic for tests on all 32-bit powerpc targets

In addition to the existing powerpc targets, powerpc64 needs libatomic
for 64-bit atomics when testing the 32-bit multilib with -m32. Adjust
the existing target checks to match all 32-bit powerpc targets, but not
64-bit ones.

libstdc++-v3/ChangeLog:

	* testsuite/lib/dg-options.exp (add_options_for_libatomic):
	Replace powerpc-ibm-aix* and powerpc*-*-darwin* with check for
	powerpc && ilp32.