build: clang support

build: clang support

This works with at least one version of clang
that existed at one moment in time.
No guarantees about clangs past or future.

To try:
        CC=clang all.bash

It does not work with the Xcode clang,
because that clang fails at printing a useful answer
to:
        clang -print-libgcc-file-name
The clang that works prints a full path name for
that command, not just "libgcc.a".

Fixes #4713.

R=iant, minux.ma
CC=golang-dev
https://golang.org/cl/7323068