Set pcpu curpmap for powerpc64

Summary:
If an illegal instruction is encountered on a process running on a
powerpc64 kernel it would attempt to sync the cache before retrying the
instruction "just in case". However, since curpmap is not set, when
moea64_sync_icache() attempts to lock the pmap, it's locking on a NULL pointer,
triggering a panic. Fix this by adding a (assumed unnecessary) fallback to
curthread's pmap in moea64_sync_icache().

Reported by: alfredo.junior_eldorado.org.br
Reviewed by: luporl, alfredo.junior_eldorado.org.br
Differential Revision: https://reviews.freebsd.org/D20911