powerpc64: Micro-optimize moea64 native pmap tlbie

powerpc64: Micro-optimize moea64 native pmap tlbie

* Cache moea64_need_lock in a local variable; gcc generates slightly better
  code this way, it doesn't need to reload the value from memory each read.
* VPN cropping is only needed on PowerPC ISA 2.02 and older cores, a subset
  of those that need serialization, so move this under the need_lock check,
  so those that don't need the lock don't even need to check this.