powerpc: Don't use -mno-strict-align on clang

We added -mno-strict-align in commit f036b368

 (powerpc: Work around little
endian gcc bug) to fix gcc bug http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57134

Clang doesn't understand it. We need to use a conditional because we can't use the
simpler call cc-option here.

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