[ARC] Improve parsing instruction operands.

[ARC] Improve parsing instruction operands.

We use gas' expression function to parse the operands of an
instruction in a generic way. There are situations when we have labels
and registers having the same name as well as the substraction sign
doesn't always stands for the arithmetical operation but for the
register range (e.g. enter instruction). This patch improves parsing
symbols found in a instruction operand, cleans up code and avoids
using default or undefined variables.

gas/
xxxx-xx-xx  Claudiu Zissulescu  <claziss@synopsys.com>

	* config/tc-arc.c (parse_reloc_symbol): New function.
	(tokenize_arguments): Clean up, use parse_reloc_symbol function.
	(md_operand): Set X_md to absent.
	(arc_parse_name): Check for X_md.