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.