MACHINE_CPU is not defined on powerpc*, we rely only on ARCH.
Because of this we choose to optimize using instructions available in
POWER8 and newer, so this port built with SIMD option won't work on older
POWER CPUs (we could however add additional options for that, to support
AltiVec in older POWER, if desired).
While here, also remove BROKEN on powerpc.
PR: 234953
Submitted by: Piotr Kubaj
Approved by: portmgr (tier-2 blanket)
9400c25494