RISC-V: Add fence.tso instruction

RISC-V: Add fence.tso instruction

The RISC-V memory model has been ratified, and it includes an additional
fence: "fence.tso".  This pseudo instruction extends one of the
previously reserved full fence patterns to be less restrictive, and
therefor will execute correctly on all existing microarchitectures.
Thus there is no reason to allow this instruction to be disabled (or
unconverted to a full fence), so it's just unconditionally allowed.

I've added a test case for GAS to check that "fence.tso" correctly
assembles on rv32i-based targets.  I checked to see that "fence.tso"
appears in "gas.log", but that's the only testing I've done.

gas/ChangeLog

2018-10-02  Palmer Dabbelt  <palmer@sifive.com>

        * testsuite/gas/riscv/fence-tso.d: New file.
        * testsuite/gas/riscv/fence-tso.s: Likewise.

include/ChangeLog

2018-10-02  Palmer Dabbelt  <palmer@sifive.com>

        * opcode/riscv-opc.h (MATCH_FENCE_TSO): New define.
        (MASK_FENCE_TSO): Likewise.

opcodes/ChangeLog

2018-10-02  Palmer Dabbelt  <palmer@sifive.com>

        * riscv-opc.c (riscv_opcodes) <fence.tso>: New opcode.