cmd/go: accept build flags in clean and list

cmd/go: accept build flags in clean and list

list has been adding them one at a time haphazardly
(race and tags were there and documented; compiler
was there and undocumented).

clean -i needs -compiler in order to clean the
installed targets for alternate compilers.

Fixes #7302.

While we're here, tweak the language in the 'go get' docs
about build flags.

Fixes #7807.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/99130043