target/riscv: rvv-1.0: trigger illegal instruction exception if frm is not valid

If the frm field contains an invalid rounding mode (101-111),
attempting to execute any vector floating-point instruction, even
those that do not depend on the rounding mode, will raise an illegal
instruction exception.

Call gen_set_rm() with DYN rounding mode to check and trigger illegal
instruction exception if frm field contains invalid value at run-time
for vector floating-point instructions.

Signed-off-by: Frank Chang <frank.chang@sifive.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211210075704.23951-68-frank.chang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>