cmd/go: rewrite paths for overlaid files using -trimpath

Pass the trimpath flag to cmd/compile to use the correct file paths
for files that are overlaid: that is, the "destination" path in the
overlay's Replace mapping rather than the "source" path.

Also fix paths to go source files provided to the gccgo compiler.

For #39958

Change-Id: I3741aeb2272bd0d5aa32cb28133b61e58264fd39
Reviewed-on: https://go-review.googlesource.com/c/go/+/257198
Trust: Michael Matloob <matloob@golang.org>
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

0 files changed, 0 insertions, 0 deletions