metag: make an array's type unsigned char[]

metag: make an array's type unsigned char[]

It contains values between 128 and 256 which fit in an unsigned char, but not a
signed char, so we should explicitly use unsigned char to not rely on how these
values are converted to signed char.

gas/ChangeLog:

2016-05-26  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* config/tc-metag.c (metag_handle_align): Make the type of noop
	unsigned char.