gccrs: Fix the parser's operand and flags storage

gccrs: Fix the parser's operand and flags storage

gcc/rust/ChangeLog:

	* ast/rust-expr.h (struct InlineAsmOperand):
	Add construction for register_type
	* expand/rust-macro-builtins-asm.cc (parse_reg_operand):
	Fix parsing logic & reassignment logic
	(parse_reg_operand_in): Fix parsing
	(parse_reg_operand_out): Fix parsing
	(parse_reg_operand_inout): Fix parsing
	(parse_reg_operand_unexpected): Remove rust_unreachable()
	(parse_asm_arg): Fix parsing logic
	* expand/rust-macro-builtins-asm.h: Add = operator overloading

gcc/testsuite/ChangeLog:

	* rust/compile/inline_asm_illegal_operands.rs: Test now passing
	* rust/compile/inline_asm_parse_operand.rs: Remove _, not
	supported right now