Remove GOBIN in PATH dependency; don't assume cwd is $GOROOT/src

Remove GOBIN in PATH dependency; don't assume cwd is $GOROOT/src

This change removes the necessity to have GOBIN in $PATH,
and also doesn't assume that the build is being run from
$GOROOT/src. This is a minimal set of necessary changes
to get Go to build happily from the FreeBSD ports
collection.

R=rsc
CC=golang-dev
https://golang.org/cl/171044