Merge a rudimentary gxemul "oldtestmips" port. This consists almost

Merge a rudimentary gxemul "oldtestmips" port.  This consists almost
entirely of one machdep file lifted from the MALTA port, as well as
a low-level console and tty driver for the gxemul debugging console
device (the emulators stdio).  As with many low-level embedded and
hypervisor console devices, it is polled only, so we drive TTY I/O
from a callout; we are perhaps a bit too aware of the MIPS physical
maps in order to attach the console before newbus comes to life.

The sample kernel configuration depends on an MD-based root file
system, which is not provided.  However, any 64-bit, big-endian
userspace image (such as one generated for MALTA) should work.

This will hopefully be supplemented by additional device drivers for
gxemul-specific hardware simulations from Juli Mallett.  We have
found oldtestmips quite useful for testing and improving aspects of
the MIPS port, so it's worth supporting better in FreeBSD.

Requested by:	theraven, jmallett
Sponsored by:	DARPA, AFRL
MFC after:	3 weeks