powerpc/dtrace: Guard fuword64 usage for powerpc64 only

32-bit powerpc doesn't have a fuword64, so only use it on powerpc64.
This could also be done elsewhere that splits 32-bit and 64-bit
accesses, but adding ifdefs everywhere would just clutter up the source
for little benefit. This is the only usage of fuword64 directly; other
archs have a dtrace_fuword64_nocheck(), but powerpc does not, and I
don't see a need to add it currently.

MFC after: 1 week