* 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.
config files moved to an appropriate config/<cpu> subdirectory.
* nm-*, xm-*, tm-*: All native, host, and target files, which
get linked to nm.h, xm.h, and tm.h respectively by configure,
moved to appropriate config/<cpu> subdirectory.