target/riscv: rvv-1.0: update opivv_vadc_check() comment

Vector Integer Add-with-Carry / Subtract-with-Borrow Instructions is
moved to Section 11.4 in RVV v1.0 spec. Update the comment, no
functional changes.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211210075704.23951-77-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>