target/riscv: rvv-1.0: single-width floating-point reduction

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211210075704.23951-54-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>