RISC-V: Add missing privileged spec registers.

RISC-V: Add missing privileged spec registers.

	gas/
	* testsuite/gas/riscv/priv-reg.d, testsuite/gas/riscv/priv-reg.s: New.

	include/
	* opcode/riscv-opc.h (DECLARE_CSR): Add missing privileged registers.
	Sort to match privileged spec documentation order.
	(DECLARE_CSR_ALIAS): Add ubadaddr, and comments.