cmd/go: gccgo: consistent results, fix vendoring
cmd/go: gccgo: consistent results, fix vendoring

Pass the -fdebug-prefix-map and -gno-record-gcc-switches compiler
options to gccgo to generate consistent results.

Fix the vendoring code to look for /vendor/, not just /vendor, to
avoid being confused by something like vendor/vendor.org.

Tested by the cmd/go tests in a followup CL.

Change-Id: I814b38ef4d82fd0fa1bec4c1ec743ea1e717711f
Reviewed-on: https://go-review.googlesource.com/45695
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

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