Update mn10300 dwarf register map

This change updates the mn10300 dwarf register map.  It reduces the
failure count when doing simulator testing against the default
multilib from 788 to 99.

gdb/ChangeLog:

	* mn10300-tdep.c (mn10300_dwarf2_reg_to_regnum): Rewrite
	dwarf2_to_gdb[] table using symbolic constants.  Adjust
	penultimate entry from number representing the PC register
	to symbolic constant representing the MDR register.  Add
	constant for the PC register to the end of the table.