Add support for handling undefined instructions in userspace and the
kernel. We can register callbacks to perform the required operation on the
saved registers before returning.

This is initially used to work around a bug in old versions of QEMU that
trigger such an exception when reading from an ID register when it should
load z zero value.

I expect this could be used with other exception types, e.g. to emulate
special register access from userland.

Sponsored by: DARPA, AFRL