cmd/go: make file:line for cgo files look like non-cgo files

cmd/go: make file:line for cgo files look like non-cgo files

Passing the absolute path to cgo puts the absolute path in the
generated file's //line directives, which then shows that path
in the compiler output, which the go command can then
make relative to the current directory, same as it does for
other compiler output.

Change-Id: Ia2064fea40078c46fd97e3a3b8c9fa1488f913e3
Reviewed-on: https://go-review.googlesource.com/77154
Reviewed-by: Ian Lance Taylor <iant@golang.org>