target/riscv: zfh: implement zfhmin extension

target/riscv: zfh: implement zfhmin extension

Zfhmin extension is a subset of Zfh extension, consisting only of data
transfer and conversion instructions.

If enabled, only the following instructions from Zfh extension are
included:
  * flh, fsh, fmv.x.h, fmv.h.x, fcvt.s.h, fcvt.h.s
  * If D extension is present: fcvt.d.h, fcvt.h.d

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20211210074329.5775-8-frank.chang@sifive.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>