target/riscv: Introduce helper_set_rounding_mode_chkfrm

target/riscv: Introduce helper_set_rounding_mode_chkfrm

The new helper always validates the contents of FRM, even
if the new rounding mode is not DYN.  This is required by
the vector unit.

Track whether we've validated FRM separately from whether
we've updated fp_status with a given rounding mode, so that
we can elide calls correctly.

This partially reverts d6c4d3f2a69 which attempted the to do
the same thing, but with two calls to gen_set_rm(), which is
both inefficient and tickles an assertion in decode_save_opc.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1441
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230115160657.3169274-2-richard.henderson@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>