target/arm: Make isar_feature_aa32_fp16_arith() handle M-profile

The M-profile definition of the MVFR1 ID register differs slightly
from the A-profile one, and in particular the check for "does the CPU
support fp16 arithmetic" is not the same.

We don't currently implement any M-profile CPUs with fp16 arithmetic,
so this is not yet a visible bug, but correcting the logic now
disarms this beartrap for when we eventually do.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200910173855.4068-6-peter.maydell@linaro.org