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

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

Add the following instructions:

* vfwcvt.rtz.xu.f.v
* vfwcvt.rtz.x.f.v

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

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211210075704.23951-63-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>