This also changes the bootstrap source URL to the official GitHub
mirror, to make building easier for places where Google sites aren't
accessible. Fixes #6326.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

From golang.org:
The Go programming language is an open source project to make
programmers more productive.

This commit consists of two "parts":

* golang/host: Main Go compiler for host (installed to
  STAGING_DIR_HOST/lib/go-cross), used to cross-compile Go programs to
  be packaged.

* golang (and golang-src/golang-doc): Main Go compiler for on-target
  development. These packages are quite large, but I would expect only
  developers to install these.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>