arc gas: don't use X_md as input to expression()

arc gas: don't use X_md as input to expression()

tc-arc.c:tokenize_arguments tweaks expression() parsing, controlling
whether arc_parse_name does anything by setting X_op and X_md in the
expressionS argument passed to expression().  I want to change expr()
to always fully initialise its result, and that means either a special
expression initialiser for arc, or controlling arc_parse_name by some
other means.  Since arc_parse_name already tests "assembling_insn" and
no other code does, change "assembling_insn" to directly control
arc_parse_name.  Doing it this way also stops a possible uninitialised
access to right.X_op from expr() in arc_parse_name with current gas.
The next patch in this series will also stop such uninitialised
accesses.

	* config/tc-arc.c (assembling_insn): Update comment.
	(tokenize_arguments): Don't set X_op and X_md to control
	expression(), instead just use assembling_insn.
	(md_operand): Similarly.
	(arc_parse_name): Don't test X_op and X_md.
	(md_assemble): Don't set assembling_insn here.