powerpc/boot: Use clang when CC is clang

The gcc compiler may not be available if CC is clang.

Signed-off-by: Bill Wendling <morbo@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201120224034.191382-3-morbo@google.com