Add a trap on invalid physical address access

[arichardson]

I spent a lot of time looking at instruction traces when debugging a baremetal newlib null pointer dereference. Writes to the pointer location look sensible in the instruction trace but a read returns NULL. It turned out that this null pointer dereference happended because a pointer ended up pointing past the end of physical memory.

    Memory Write [ffffffff88001d88] = ffffffff80020a98
...
...
    Memory Read [ffffffff88001d88] = 0000000000000000

It would be great if QEMU could cause a trap (maybe ADDRS/ADDRL? or even better a different exception code so we can diagnose it easily) whenever a memory address is read/written that is unmapped. If this will cause issues when running CheriBSD maybe just logging it in the textual trace/stderr would be great. Suggesting to run info mtree would also be nice.