* config/none/{nm-none.h, tm-none.h, xm-none.h}: New files.
Currently only tm-none.h has any meaningful contents.
* config/none/none.mh (NAT_FILE): Use nm-none.h
* config/none/none.mh (XM_FILE): Use xm-none.h
* config/none/none.mt (TM_FILE): Use tm-none.h
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.