cmd/go: generate correct vendor paths with -compiler gccgo option
Curently the vendor paths are not always searched for imports if
the compiler is gccgo. This change generates the vendor paths
and adds them with -I as arguments to the gccgo compile.
Fixes #15628
Change-Id: I318accbbbd8e6af45475eda399377455a3565880
Reviewed-on: https://go-review.googlesource.com/40432
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@golang.org>