target-arm: Ignore most exceptions from scalbn when doing fixpoint conversion

The VFP fixed point conversion helpers first call float_scalbn and
then convert the result to an integer. This scalbn operation may
set floating point exception flags for:
 * overflow & inexact (if it overflows to infinity)
 * input denormal squashed to zero
 * output denormal squashed to zero
Of these, we only care about the input-denormal flag, since
the output of the whole scale-and-convert operation will be
an integer (so squashed-output-denormal and overflow don't
apply). Suppress the others by saving the pre-scalb exception
flags and only copying across a potential input-denormal flag.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
