cmd/dist: make clang the default CC on FreeBSD

[ebfe]

Since FreeBSD 10 gcc is no longer part of the base system. Running ./all.bash
without explicitly set CC fails due to gcc missing (unless gcc is installed
from ports and a symlink is made). As most of the freebsd builders seem to use
CC=clang it might as well be the default.

[bradfitz]

Yeah, we could probably (at least on FreeBSD and other BSDs) fall back to CC=clang if CC isn't set and gcc isn't in $PATH.

/cc @ianlancetaylor too

[rsc]

This would fix #9108 too.

[rsc]

Too late for Go 1.5 though.

[gopherbot]

CL https://golang.org/cl/16635 mentions this issue.