sim: ppc: merge System V semaphores checks

Compile tests can use earlier defines, so hoist the HAVE_UNION_SEMUN
define to before the semaphore check, and use it in the test so that
we can merge the 2 versions into one.

This also defines HAVE_UNION_SEMUN even when ac_cv_sysv_sem is not
set, but that's OK as this define is only about a type existing, not
about whether the overall code is usable.