cmd/asm, cmd/cgo, cmd/compile, cmd/cover, cmd/link: use standard -V output

cmd/asm, cmd/cgo, cmd/compile, cmd/cover, cmd/link: use standard -V output

Also add -V=full to print a unique identifier of the specific tool being invoked.
This will be used for content-based staleness.

Also sort and clean up a few of the flag doc comments.

Change-Id: I786fe50be0b8e5f77af809d8d2dab721185c2abd
Reviewed-on: https://go-review.googlesource.com/68590
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>