New functions.
(mips_store_word): Change calling convention to return errors, and
to provide old contents if the caller wants it.
(mips_xfer_memory): Deal with errors from mips_store_word.
* config/mips/tm-idt.h, config/mips/tm-idtl.h: Remove BREAKPOINT
define now that remote-mips.c doesn't use BREAKPOINT.
* config/mips/idtl.mt: Use tm-idtl.h instead of tm-mips.h.
* config/mipos/tm-idt.h, config/mips/tm-idtl.h: New files; use
different BREAKPOINT value for IDT.