cmd/compile: add internal/runtime/atomic.Xchg8 intrinsic for PPC64

cmd/compile: add internal/runtime/atomic.Xchg8 intrinsic for PPC64

This is minor extension of the existing support for 32 and
64 bit types.

For #69735

Change-Id: I6828ec223951d2b692e077dc507b000ac23c32a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/617496
Reviewed-by: Rhys Hiltner <rhys.hiltner@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>