powerpc: Don't use gcc specific options on clang

We have code to choose between several options, eg. -mabi=elfv2 vs
-mcall-aixdesc, and -mcmodel=medium vs -mminimal-toc. But these are all
GCC specific, so use cc-option on all of them.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>