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>

src/cmd/compile/internal-abi.md

@@ -402,9 +402,6 @@ Special-purpose registers are as follows:
 | R15 | GOT reference temporary | Fixed if dynlink |
 | X15 | Zero value | Fixed |
 
-TODO: We may start with the existing TLS-based g and move to R14
-later.
-
 *Rationale*: These register meanings are compatible with Go’s
 stack-based calling convention except for R14 and X15, which will have
 to be restored on transitions from ABI0 code to ABIInternal code.