cmd/compile: replace GOROOT in //line directives

cmd/compile: replace GOROOT in //line directives

The compiler replaces any path of the form /path/to/goroot/src/net/port.go
with GOROOT/src/net/port.go so that the same object file is
produced if the GOROOT is moved. It was skipping this transformation
for any absolute path into the GOROOT that came from //line directives,
such as those generated by cmd/cgo.

Fixes #21373
Fixes #21720
Fixes #21825

Change-Id: I2784c701b4391cfb92e23efbcb091a84957d61dd
Reviewed-on: https://go-review.googlesource.com/63693
Run-TryBot: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>