PowerPC: implement md_operand to parse register names

PowerPC: implement md_operand to parse register names

Allows register names to appear in symbol assignments, so for example
 tocp = %r2
 mr %r3,tocp
now assembles.

	* gas/config/tc-ppc.c (REG_NAME_CNT): Delete, replace uses with
	ARRAY_SIZE.
	(register_name): Rename to..
	(md_operand): ..this.  Only handle %reg.
	(cr_names): Rename to..
	(cr_cond): ..this.  Just keep conditions.
	(ppc_parse_name): Add mode param.  Search both cr_cond and
	pre_defined_registers.  Handle absolute and register symbol
	values here rather than in expr.c:operand().
	(md_assemble): Don't special case register name matching in
	operands, except to set cr_operand as appropriate.
	* gas/config/tc-ppc.h (md_operand): Don't define.
	(md_parse_name, ppc_parse_name): Update.
	* read.c (pseudo_set): Copy over entire O_register value.
	* testsuite/gas/ppc/regsyms.d.
	* testsuite/gas/ppc/regsyms.s: New test.
	* testsuite/gas/ppc/ppc.exp: Run it.