powerpc: Fix -mcpu= options for SPE-only compiler

powerpc: Fix -mcpu= options for SPE-only compiler

GCC for Debian's "powerpcspe" architecture only supports 32-bit
SPE targets, and using -mcpu=powerpc or -mcpu=powerpc64 is a fatal
error.

* Change the test for a biarch compiler to pass both the -m32 and -m64
  options, so that it doesn't catch 32-bit-only compilers
* Add an ifdef CONFIG_PPC64 around the 64-bit CPU option definitions

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Topic bugfix/powerpc
Gbp-Pq: Name powerpc-fix-mcpu-options-for-spe-only-compiler.patch