[PS4][clang] Fix -funified-lto/-flto-unit default on PS4

If -funified-lto was specified but -flto was not, we would end up with a
situation where the driver thought that Unified LTO was enabled, but the
flag was not passed to the frontend. The check disabling -flto-unit for
PS4 should use the correct method to check for the target. This only
effects PS targets.