target/riscv: fix rsub gvec tcg_assert_listed_vecop assertion

gvec should provide vecop_list to avoid:
"tcg_tcg_assert_listed_vecop: code should not be reached bug" assertion.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200710104920.13550-2-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>