dev-lang/go: sync 1.13.6 with live

dev-lang/go: sync 1.13.6 with live

This syncs the following build-time changes from the live ebuild:

- use dev-lang/go-bootstrap to bootstrap the initial installation. This
  means that we will only download the relevant bootstrap archive.
  go-bootstrap can be depcleaned after the initial go installation.

- use go itself to bootstrap if it is already installed.
  This means go-bootstrap will not be downloaded as long as go is
  on the system.

- Drop the gccgo and system-bootstrap use flags.
  Bootstrapping go with gccgo had several issues, and it is no longer
  needed. Also, the system-bootstrap use flag is no longer needed since
  this behavior is the default if go is installed.

closes: https://bugs.gentoo.org/618794
closes: https://bugs.gentoo.org/664048
closes: https://bugs.gentoo.org/684652

Signed-off-by: William Hubbs <williamh@gentoo.org>