kgdb: continue and warn on signal passing from gdb

On some architectures for the segv trap, gdb wants to pass the signal
back on continue.  For kgdb this is not the default behavior, because
it can cause the kernel to crash if you arbitrarily pass back a
exception outside of kgdb.

Instead of causing instability, pass a message back to gdb about the
supported kgdb signal passing and execute a standard kgdb continue
operation.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>