cmd/link: add trampolines for too far calls in ppc64x

cmd/link: add trampolines for too far calls in ppc64x

This change adds support for trampolines on ppc64x when using
internal linking, in the case where the offset to the branch
target is larger than what fits in the field provided by the
branch instruction.

Fixes #16665

Change-Id: Icfee72910f38c94588d2adce517b64dee6176145
Reviewed-on: https://go-review.googlesource.com/30850
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>