Add SFENCE.VMA instruction

Add SFENCE.VMA instruction

include/ChangeLog:

2017-02-14  Andrew Waterman  <andrew@sifive.com>

        * opcode/riscv-opc.h (MATCH_SFENCE_VMA): New define.
        (MASK_SFENCE_VMA): Likewise.
        (sfence_vma): Declare instruction.

opcodes/ChangeLog:

2017-02-14  Andrew Waterman  <andrew@sifive.com>

        * riscv-opc.c (riscv_opcodes): Add sfence.vma instruction and
        pseudoinstructions.