cmd/compile: make math/bits.RotateLeft{32,64} intrinsics on ppc64x

Extends CL 132435 to ppc64x. ppc64x has 32- and 64-bit variable
rotate left instructions.

name             old time/op  new time/op  delta
RotateLeft32-16  1.39ns ± 0%  1.37ns ± 0%  -1.44%  (p=0.008 n=5+5)
RotateLeft64-16  1.35ns ± 0%  1.32ns ± 0%  -2.22%  (p=0.008 n=5+5)

Updates #17566

Change-Id: I567f634ff90d0691db45df0a25c99fcdfe10ca00
Reviewed-on: https://go-review.googlesource.com/c/go/+/163760


Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>