[dev.regabi] cmd/compile: remove unneeded packages from ir.Pkgs

ir.Pkgs.Itablink isn't used anymore. (I don't recall what it was ever
used for.)

ir.Pkgs.Race and ir.Pkgs.Msan are only needed in exactly only place,
so just create them on demand there, the same way that we create
"main" on demand.

Change-Id: I3474bb949f71cd40c7a462b9f4a369adeacde0d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/284230


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>