Make fixup fx_where unsigned

Another field that only stores unsigned values.

	* write.h (struct fix <fx_where>): Make unsigned.
	(fix_new, fix_at_start, fix_new_exp): Adjust prototypes.
	* write.c (fix_new, fix_new_exp, fix_at_start): Make "where" and
	"size" parameters unsigned long.
	(fix_new_internal): Likewise.  Adjust error format string to suit.
	* config/tc-mips.c (md_convert_frag): Remove cast of fx_where.
	* config/tc-sparc.c (md_apply_fix): Likewise.
	* config/tc-score.c (s3_convert_frag): Adjust for unsigned fx_where.
	* config/tc-score7.c (s7_convert_frag): Likewise.