[dev.regabi] cmd/compile: update mkbuiltin.go and re-enable TestBuiltin

Update's mkbuiltin.go to match builtin.go after the recent rf
rewrites.

Change-Id: I80cf5d7c27b36fe28553406819cb4263de84e5ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/279952


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>