Add support for the Q extension to the RISCV ISA.

Add support for the Q extension to the RISCV ISA.

gas    * config/tc-riscv.c (riscv_set_arch): Whitelist the "q" ISA
        extension.
        (riscv_after_parse_args): Set FLOAT_ABI_QUAD when the Q ISA is
        enabled and no other ABI is specified.

include * opcode/riscv-opc.h: Add support for the "q" ISA extension.

opcodes * riscv-opc.c (riscv-opcodes): Add support for the "q" ISA
        extension.
        * riscv-opcodes/all-opcodes: Likewise.