Disconnect fasttrap from the 32-bit powerpc build.

An upcoming bug fix requires 64-bit atomics, which aren't implemented on
powerpc. The powerpc port of fasttrap is incomplete anyway and doesn't
get loaded by dtraceall.ko on powerpc because of a missing dependency;
it's presumed that it's effectively unused.

Discussed with: jhibbits
MFC after: 2 weeks