cmd/compile: assert that function values reference ABIInternal

Function values must always point to the ABIInternal entry point of a
function. It wasn't entirely obvious to me we were getting this right,
so this CL adds checks for this.

Updates #40724.

Change-Id: Idd854e996d63d9151c28ec5c9251b690453b1024
Reviewed-on: https://go-review.googlesource.com/c/go/+/305272


Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>