cmd/go: bring in final version of gccgo pkg-config support
cmd/go: bring in final version of gccgo pkg-config support

This updates gccgo to the final version of https://golang.org/cl/18790,
by Michael Hudson-Doyle.

Update golang/go#11739.

Change-Id: I66b42d6dbe6ed2ea8b0e7cbdd1ff211704811614
Reviewed-on: https://go-review.googlesource.com/22400
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend