Use gdbarch_pc_regnum instead of gdbarch_write_pc.

gdb/ChangeLog:

	* m32r-tdep.c (m32r_write_pc): Delete.
	(m32r_gdbarch_init): Remove call to set_gdbarch_write_pc.
	Add call to set_gdbarch_pc_regnum.
mep: define gdbarch_pc_regnum instead of gdbarch_write_pc.

gdb/ChangeLog:

	* mep-tdep.c (mep_write_pc): Delete.
	(mep_gdbarch_init): Remove call to set_gdbarch_write_pc.
	Add call to set_gdbarch_pc_regnum.