Disconnect powerpc from the default tinderbox for now.

The wrong toolchain was set in MAKE_PARAMS_powerpc, however, there are
some other issues preventing powerpc from building in tinderbox:

1. There is no powerpc-gcc and powerpc-xtoolchain-gcc port that provides an external 32-bit powerpc GCC toolchain.

2. On other targets, the same toolchain can build all of the architectures for a given target. MIPS achieves this by always setting -mabi and -EB/-EL explicitly instead of relying on the compiler's default architecture. PowerPC might be able to do the same thing, but as of today, powerpc-gcc would be required for powerpc and powerpcspe and powerpc64-gcc would be required for powerpc64. Our existing logic for make universe does not permit per-MACHINE_ARCH toolchains.

I tried hacking TARGETS_powerpc to only include powerpc64 when
powerpc64-gcc was present, and while that skipped the 32-bit worlds,
it tried to build all the kernels.

Reported by: jeff
Discussed with: imp