[PATCH] allow kernel compile with native ppc64 compiler

[Paul Mackerras]

Olaf Hering writes:

> The zImage is a 32bit binary, but a native powerpc64-linux gcc will
> produce 64bit objects in arch/ppc64/boot.
> This patch fixes it.

... and breaks the compile on older toolchains that don't understand
-m32.  We need to make the -m32 conditional on HAS_BIARCH as defined
in arch/ppc64/Makefile.

Paul.