cmd/compile: intrinsify math.MulUintptr on PPC64

cmd/compile: intrinsify math.MulUintptr on PPC64

This can be done efficiently with few instructions.

This also adds MULHDUCC for further codegen improvement.

Change-Id: I06320ba4383a679341b911a237a360ef07b19168
Reviewed-on: https://go-review.googlesource.com/c/go/+/605975
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Archana Ravindar <aravinda@redhat.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>