target/riscv: change the api for RVF/RVD fmin/fmax

target/riscv: change the api for RVF/RVD fmin/fmax

The sNaN propagation behavior has been changed since cd20cee7 in
https://github.com/riscv/riscv-isa-manual.

In Priv spec v1.10, RVF is v2.0. fmin.s and fmax.s are implemented with
IEEE 754-2008 minNum and maxNum operations.

In Priv spec v1.11, RVF is v2.2. fmin.s and fmax.s are amended to
implement IEEE 754-2019 minimumNumber and maximumNumber operations.

Therefore, to prevent the risk of having too many version variables.
Instead of introducing an extra *fext_ver* variable, we tie RVF version
to Priv version. Though it's not completely accurate but is close enough.

Signed-off-by: Chih-Min Chao <chihmin.chao@sifive.com>
Signed-off-by: Frank Chang <frank.chang@sifive.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211021160847.2748577-3-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>