cmd/compile/internal-abi: update internal ABI spec for g register

We've already implemented dedicating R14 as the G register on amd64,
so remove the TODO saying we might want to hold off on this.

For #40724.

Change-Id: I45b24ced03cac862127b53f5e9a4b4bcf6b1f86c
Reviewed-on: https://go-review.googlesource.com/c/go/+/295790


Trust: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>