target/arm: make whpx use syndrome helpers for decode

Rather than open coding a bunch of shifts and masks we can use the
syndrome definitions. While we are at it assert it really is a
EC_DATAABORT.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20260422125250.1303100-19-alex.bennee@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>