[dev.regabi] cmd/compile: change ir.NameOffsetExpr to use *obj.LSym instead of *Name

Because NameOffsetExpr is always used with global variables, and SSA
backend only needs (*Name).Linksym() to generate value for them.

Passes toolstash -cmp.

Updates #43737

Change-Id: I17209e21383edb766070c0accd1fa4660659caef
Reviewed-on: https://go-review.googlesource.com/c/go/+/284119


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