Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

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