sh: kgdb: Fill out sleeping_thread_to_gdb_regs() state.

Presently we're using a pretty dumbed-down implementation that copies
over register state visible from the thread info, leaving the bulk of the
switch_to state uncopied.

Given that we're also depending on register bank toggling for switch_to
optimization we ought to also explicitly zero out the GP regs that reside
in an alternate bank in order to prevent handing back garbage.

There are a few extra registers that we have state for in switch_to, so
copy those over while we're at it.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>