target/arm: Copy entire float_status in is_ebf

target/arm: Copy entire float_status in is_ebf

Now that float_status has a bunch of fp parameters,
it is easier to copy an existing structure than create
one from scratch.  Begin by copying the structure that
corresponds to the FPSR and make only the adjustments
required for BFloat16 semantics.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20241203203949.483774-2-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>