cmd/cgo: don't use absolute paths in the export header file

We were using absolute paths in the #line directives in the export
header file. This makes the header file change if you move GOPATH.
The absolute paths aren't helpful for the final user, which is some C
program elsewhere.

Fixes #24945

Change-Id: I2da32c9b477df578bd5087435a03fe97abe462e3
Reviewed-on: https://go-review.googlesource.com/108315


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>