cmd/go: update to match recent changes to gc

In https://golang.org/cl/111097 the gc version of cmd/go was updated
to include some gofrontend-specific changes. The gofrontend code
already has different versions of those changes; this CL makes the
gofrontend match the upstream code.

Change-Id: Idb162435fd7cf4f92a7f5eb45d5f4fe48c44caba
Reviewed-on: https://go-review.googlesource.com/111099
Reviewed-by: Than McIntosh <thanm@google.com>