crypto/ppccap.c: SIGILL-free processor capabilities detection on MacOS X.

It seems to be problematic to probe processor capabilities with SIGILL
on MacOS X. The problem should be limited to cases when application code
is debugged, but crashes were reported even during normal execution...

[backport of 0bd93bbe

]

Reviewed-by: Rich Salz <rsalz@openssl.org>