target/arm: Fix has_vfp/has_neon ID reg squashing for M-profile

In arm_cpu_realizefn(), if the CPU has VFP or Neon disabled then we
squash the ID register fields so that we don't advertise it to the
guest.  This code was written for A-profile and needs some tweaks to
work correctly on M-profile:

 * A-profile only fields should not be zeroed on M-profile:
   - MVFR0.FPSHVEC,FPTRAP
   - MVFR1.SIMDLS,SIMDINT,SIMDSP,SIMDHP
   - MVFR2.SIMDMISC
 * M-profile only fields should be zeroed on M-profile:
   - MVFR1.FP16

In particular, because MVFR1.SIMDHP on A-profile is the same field as
MVFR1.FP16 on M-profile this code was incorrectly disabling FP16
support on an M-profile CPU (where has_neon is always false).  This
isn't a visible bug yet because we don't have any M-profile CPUs with
FP16 support, but the change is necessary before we introduce any.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20201019151301.2046-9-peter.maydell@linaro.org

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