RISC-V/zfh: Add half-precision floating-point v0.1 instructions.

RISC-V/zfh: Add half-precision floating-point v0.1 instructions.

This patch is porting from the following riscv github,
https://github.com/riscv/riscv-binutils-gdb/commits/rvv-1.0.x-zfh

And here is the draft zfh spec,
https://github.com/riscv/riscv-isa-manual/tree/zfh

bfd/
    * elfxx-riscv.c (riscv_std_draft_ext_strtab): Added zfh.
    (riscv_implicit_subset): Add implicit f and zicsr for zfh.
gas/
    * config/tc-riscv.c (extended_ext_version_table): Added default
    version of zfh to v0.1.
    (riscv_extended_subset_supports): Handle INSN_CLASS*_ZFH.
    (extended_macro): Handle M_FLH and M_FSH.
    * testsuite/gas/riscv/extended/extended.exp: Updated.
    * testsuite/gas/riscv/extended/fp-zfh-insns.d: New testcase.
    * testsuite/gas/riscv/extended/fp-zfh-insns.s: Likewise.
include/
    * opcode/riscv-opc-extended.h: Added zfh encoding macros
    and DECLARE_INSN.
    * opcode/riscv.h (enum riscv_extended_insn_class): Added INSN_CLASS*_ZFH.
    (enum M_FLH, M_FSH): Added.
opcodes/
    * riscv-opc.c (riscv_draft_opcodes): Added zfh instructions.