cmd/link/internal/arm64: support Mach-O ARM64_RELOC_POINTER_TO_GOT in internal linking

cmd/link/internal/arm64: support Mach-O ARM64_RELOC_POINTER_TO_GOT in internal linking

ARM64_RELOC_POINTER_TO_GOT is the arm64 version of X86_64_RELOC_GOT, which has been support
for many years now.

The standard library still doesn't need it, but I've found it necessary
when statically linking against a library I own.

Change-Id: I8eb7bf3c74aed663a1fc00b5dd986057130f7f7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/703315
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>