cmd/link/internal/ppc64: generate PCrel trampolines on P10

cmd/link/internal/ppc64: generate PCrel trampolines on P10

PCrelative trampolines have no dependence on the TOC pointer or build
mode, thus they are preferable to use when supported.

This is a step towards eliminating the need to use and maintain the
TOC pointer in R2 when PCrel is supported.

Change-Id: I1b1a7e16831cfd6732b31f7fad8df2a7c88c8f84
Reviewed-on: https://go-review.googlesource.com/c/go/+/461599
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>