mpr/mps crash badly. Part of the stability added use of atomic64 functions not
present on 32-bit powerpc. Merge the part that removes mps from the build
on this tier 2 platform. Working mpr/mps in 12 and 12.1 on our tier 1 platforms
is more important.

MFC r341754:

Remove the mps driver from powerpc 32bit GENERIC, and don't build it and mpr
as a module for powerpc or mips.  An upcoming commit will cause these drivers
to rely on the presence of 64bit atomic operations.  Discussed with jhibbits.

Relnotes: YES