cmd/go, cmd/vet, go/internal/gccgoimport: make vet work with gccgo

When using gccgo/GoLLVM, there is no package file for a standard
library package. Since it is impossible for the go tool to rebuild the
package, and since the package file exists only in the form of a .gox
file, this seems like the best choice. Unfortunately it was confusing
vet, which wanted to see a real file. This caused vet to report errors
about missing package files for standard library packages. The
gccgoimporter knows how to correctly handle this case. Fix this by

1) telling vet which packages are standard;
2) letting vet skip those packages;
3) letting the gccgoimporter handle this case.

As a separate required fix, gccgo/GoLLVM has no runtime/cgo package,
so don't try to depend on it (as it happens, this fixes #25324).

The result is that the cmd/go vet tests pass when using -compiler=gccgo.

Fixes #25324

Change-Id: Iba8f948fe944da5dc674f580bd3321929ee50fa0
Reviewed-on: https://go-review.googlesource.com/113716


Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>