target/arm: Adjust exception flag handling for AH = 1

target/arm: Adjust exception flag handling for AH = 1

When FPCR.AH = 1, some of the cumulative exception flags in the FPSR
behave slightly differently for A64 operations:
 * IDC is set when a denormal input is used without flushing
 * IXC (Inexact) is set when an output denormal is flushed to zero

Update vfp_get_fpsr_from_host() to do this.

Note that because half-precision operations never set IDC, we now
need to add float_flag_input_denormal_used to the set we mask out of
fp_status_f16_a64.

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