Update to GCC 4.6.4, the final release and end of the GCC 4.6 branch.
(This requires an EPOCH bump due to how snapshots are labeled.)
Introduce a new option BOOTSTRAP that allows for doing a full
bootstrap of GCC, as opposed to just running a simple build.
Bootstrapping is actually the default upstream, we disable it
by default for the stable flavors of GCC since that is a huge
win in terms of build time of the port. No change in default
behavior for this port.