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.

Tested powerpc64-linux (with both -m32 and -m64) and
powerpc-ibm-aix7.2.0.0 (with both -maix32 and -maix64)
and powerpc64le-linux (-m64 only).

Committed to trunk.