rs6000: Use strlen instead of sizeof - 1

rs6000: Use strlen instead of sizeof - 1

It is easier to read and understand  strlen ("string")  than it is to
read and understand  sizeof ("string") - 1  .

	* config/rs6000/rs6000.c (rs6000_debug_print_mode): Don't use sizeof
	where strlen is more legible.
	(rs6000_builtin_vectorized_libmass): Ditto.
	(rs6000_print_options_internal): Ditto.