[dev.regabi] cmd/compile: use LinksymOffsetExpr in walkConvInterface

This CL updates walkConvInterface to use LinksymOffsetExpr for
referencing runtime.staticuint64s and runtime.zerobase.

Passes toolstash -cmp (surprisingly).

Change-Id: Iad7e30371f89c8a5e176b5ddbc53faf57012ba0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/284229


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>