Correct and expand handling of `movm' instruction, and register

Correct and expand handling of `movm' instruction, and register
saves in general.
* config/mn10300/tm-mn10300.h (D0_REGNUM, A0_REGNUM, MDRQ_REGNUM,
MCRH_REGNUM, MCRL_REGNUM, MCVF_REGNUM): New definitions.
(enum movm_register_bits): New enum.
* mn10300-tdep.c (set_movm_offsets): Use symbolic names for the
bits, not hex literals.  Handle the `other', `exreg0', and
`exother' bits.  Correct handling of `exreg1': it saves r4, r5,
r6, and r7, not r2, r3, r4, and r5.
(saved_regs_size): New function.
(mn10300_frame_chain, mn10300_frame_saved_pc): Use it, instead
of computing the same thing inline, incorrectly.