powerpc: Don't do gcc version checks if we're building with clang

We have several checks for bad gcc versions in our Makefile. These don't
apply if we're building with clang, so skip them in that case.

The obvious check would be for ${COMPILER} = "gcc", but because of the
way the logic in the top level Makefile conditionally sets COMPILER,
it's possible that we're building with gcc but COMPILER was not set.

So instead check for ${COMPILER} != "clang", which we know is currently
the only other possibility.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>