Kbuild change breaks the ppc64 build

[Paul Mackerras]

Commit 5de043f4bd11a9e0a3e8daec7d1905da575a76b7 breaks the build on
64-bit powerpc because we no longer get the -m64 flag passed to gcc.
There is code in arch/powerpc/Makefile which adds (or used to add)
-m64 to AS, LD and CC if we are running on a 64-bit machine (which I
am) and have a biarch toolchain (which I do).  Without -m64, the
toolchain assumes 32-bit and all sorts of things break spectacularly.

I haven't yet tracked down exactly why this commit has this effect,
since I find it takes considerable time and effort to understand
Kbuild.

Paul.