cris: sprintf optimisation

Since I was poking at cris-dis.c to avoid the sanitizer warning,
I figure I might as well make use of stpcpy and sprintf return value
in other places in this file.

	* cris-dis.c (format_hex): Use sprintf return value.
	(format_reg): Use stpcpy and sprintf return, avoiding strlen.
	(format_sup_reg): Likewise.