RISC-V: Add 2 missing privileged registers.

RISC-V: Add 2 missing privileged registers.

	gas/
	* testsuite/gas/riscv/priv-reg.s: Add missing stval and mtval.
	* testsuite/gas/riscv/priv-reg.d: Likewise.

	include/
	* opcode/riscv-opc.h (CSR_SBADADDR): Rename to CSR_STVAL.  Rename
	DECLARE_CSR entry.  Add alias to map sbadaddr to CSR_STVAL.
	(CSR_MBADADDR): Rename to CSR_MTVAL.  Rename DECLARE_CSR entry.
	Add alias to map mbadaddr to CSR_MTVAL.