gdb/h8300-tdep.c: Fix register name in h8300h machine.

gdb/h8300-tdep.c: Fix register name in h8300h machine.

H8/300H general register name "ER0" - "ER7".
But gdb using "R0" - "R7".
This changes register name "ER0" - "ER7" in h8300h machine mode.

gdb/ChangeLog:

	* h8300-tdep.c (h8300_register_name_common): New.
	h8300_register_name): Use h8300_register_name_common.
	(h8300s_register_name): Likewise.
	(h8300sx_register_name): Likewise.
	(h8300h_register_nam): New.
	(h8300_gdbarch_init): Use h8300h_register_name in h8300h machine.