RISC-V: Support Zalasr extension.

RISC-V: Support Zalasr extension.

This patch supports Zalasr extension(load-acquire/store-release) instructions.

https://github.com/riscv/riscv-zalasr

bfd/ChangeLog:

        * elfxx-riscv.c (riscv_multi_subset_supports): New ext.
        (riscv_multi_subset_supports_ext): Ditto.

gas/ChangeLog:

        * NEWS: Support Zalasr extension.
        * testsuite/gas/riscv/march-help.l: New ext.
        * testsuite/gas/riscv/zalasr.d: New test.
        * testsuite/gas/riscv/zalasr.s: New test.

include/ChangeLog:

        * opcode/riscv-opc.h (MATCH_LBA): New match opcode.
        (MASK_LBA): New mask opcode.
        (MATCH_LHA): New match opcode.
        (MASK_LHA): New mask opcode.
        (MATCH_LWA): New match opcode.
        (MASK_LWA): New mask opcode.
        (MATCH_LDA): New match opcode.
        (MASK_LDA): New mask opcode.
        (MATCH_SBR): New match opcode.
        (MASK_SBR): New mask opcode.
        (MATCH_SHR): New match opcode.
        (MASK_SHR): New mask opcode.
        (MATCH_SWR): New match opcode.
        (MASK_SWR): New mask opcode.
        (MATCH_SDR): New match opcode.
        (MASK_SDR): New mask opcode.
        (DECLARE_INSN): New insn declare.
        * opcode/riscv.h (enum riscv_insn_class): New ext.

opcodes/ChangeLog:

        * riscv-opc.c: New instructions def.