Log Message:
gxemul: Update to 0.6.3.1

Changelog:
20210215        Implementing the fadd.dss 88K instruction, and working a bit
                more on the exception cases in the xmem instruction.
20210220        Implementing mouse emulation support for the LUNA88K machine.
                Updating OpenBSD/sgi install instructions (6.5 -> 6.8),
                and including a screenshot of X11 running.
20210221        Handle return values from interrupted select() when checking
                for available console input. (Found when using CTRL-C on Linux
                to enter the debugger command line, and noticed that it was
                spuriously broken.)
20210222        Basic multiprocessor bootup of OpenBSD/luna88k now works, but
                it is slightly unstable.