MN10300: Make the kernel jump into gdbstub on a BUG

Make the kernel jump into gdbstub (if configured) on a BUG with the register
set from the BUG rather than interpolating another illegal instruction and
leaving gdbstub's idea of the process counter in unsupported_syscall() where
the original BUG was detected.

With this patch, gdbstub reports a SIGABRT to the compiler and reports the
program counter at the original BUG, allowing the execution state at the time
of the BUG to be examined with GDB.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>