score gcc-8 warning fixes

Rather than just silencing the gcc-8 warnings, I decided to rewrite
the buffer handling in the two functions where gcc was warning.
The rest of the file could do with the same treatment.

	* config/tc-score.c (s3_do_macro_bcmp): Don't use fixed size
	buffers.
	(s3_do_macro_bcmpz): Likewise.