cmd/compile: move Linksym, linksymname, and isblanksym to types package

cmd/compile: move Linksym, linksymname, and isblanksym to types package

Response to code review feedback on CL 40693.

This CL was prepared by:

(1) manually adding new implementations and the Ctxt var to package types

(2) running eg with template:

func before(s *types.Sym) *obj.LSym { return gc.Linksym(s) }
func after(s *types.Sym) *obj.LSym  { return s.Linksym() }

(3) running gofmt -r:

gofmt -r 'isblanksym(a) -> a.IsBlank()'

(4) manually removing old implementations from package gc

Passes toolstash-check.

Change-Id: I39c35def7cae5bcbcc7c77253e5d2b066b981dea
Reviewed-on: https://go-review.googlesource.com/41302
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>