target/arm: Implement FPCR.FIZ handling

target/arm: Implement FPCR.FIZ handling

Part of FEAT_AFP is the new control bit FPCR.FIZ.  This bit affects
flushing of single and double precision denormal inputs to zero for
AArch64 floating point instructions.  (For half-precision, the
existing FPCR.FZ16 control remains the only one.)

FPCR.FIZ differs from FPCR.FZ in that if we flush an input denormal
only because of FPCR.FIZ then we should *not* set the cumulative
exception bit FPSR.IDC.

FEAT_AFP also defines that in AArch64 the existing FPCR.FZ only
applies when FPCR.AH is 0.

We can implement this by setting the "flush inputs to zero" state
appropriately when FPCR is written, and by not reflecting the
float_flag_input_denormal status flag into FPSR reads when it is the
result only of FPSR.FIZ.

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