Add a bootstrap target to Hotspot make

The Hotspot build requires a bootstrap jdk. When doing a port, such a jdk is usually
not available, so hackery is required to bootstrap the initial build. 6526456
alleviates the problem a bit, but a more usable solution would be to add a 'bootstrap'
target to the Hotspot build that could be executed entirely on a host platform.
Then the process would be to run 'make bootstrap' using a bootstrap jdk running
on a host platform, then 'make' on the target platform.