[dev.regabi] cmd/compile: refactor Linksym creation

Currently there's a lot of logic within package types for creating
Linksyms. This CL pulls it out into base, where it can be more easily
reused by other compiler code that shouldn't need to depend on package
types.

Package base probably isn't the best place for this, but it's
convenient because it's a package that types already depends on. It's
also where the Ctxt object lives, which these functions depend upon.

Passes toolstash -cmp w/ -gcflags=all=-abiwrap.

Change-Id: I50d8b7e4596955205036969eab24d7dab053b363
Reviewed-on: https://go-review.googlesource.com/c/go/+/284231


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>