Add g10 support to rl78-tdep.c.

Add g10 support to rl78-tdep.c.

The g10 is an rl78 variant which has fewer registers.  Aside from the
obvious addition of a new register_name() function which omits
registers which the g10 lacks, this change also updates the
return_value() function with the conventions specified by the ABI for
finding and setting return values.

	* rl78-tdep.c (rl78_g10_register_name): New function.
	(rl78_return_value): Add g10 support.
	(rl78_gdbarch_init): Register rl78_g10_register_name for the
	g10.