target/riscv: rvv-1.0: floating-point/integer type-convert instructions

target/riscv: rvv-1.0: floating-point/integer type-convert instructions

Add the following instructions:

* vfcvt.rtz.xu.f.v
* vfcvt.rtz.x.f.v

Also adjust GEN_OPFV_TRANS() to accept multiple floating-point rounding
modes.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211210075704.23951-62-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>