S12Z: New option -mreg-prefix

S12Z: New option -mreg-prefix

Add a new machine dependent option to set a prefix for register names.

gas/

* config/tc-s12z.c (register_prefix): New variable.  (md_show_usage,
  md_parse_option):  parse the new option.
  (lex_reg_name): Scan the prefix if one is set.
* doc/c-s12z.texi (S12Z-Opts): Document the new option.
* testsuite/gas/s12z/reg-prefix.d: New file.
* testsuite/gas/s12z/reg-prefix.s: New file.
* testsuite/gas/s12z/s12z.exp: Add them.