[dev.regabi] cmd/compile: update mkbuiltin.go to use new type constructors

We recently added new functions to types like NewSignature and
NewField, so we can use these directly rather than depending on the
typecheck and ir wrappers.

Passes toolstash -cmp.

Change-Id: I32676aa9a4ea71892216017756e72bcf90297219
Reviewed-on: https://go-review.googlesource.com/c/go/+/279953


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>