arch-arm: Do not increment exponent if FPSCR.FZ in fplib

arch-arm: Do not increment exponent if FPSCR.FZ in fplib

If flushing to zero, the exponent shouldn't be incremented since
we are supposed to produce a 0 value and not a denormal number

Change-Id: Ib6dd594a6555b2fd9a20a52b59cbf1f5f94c2eb5
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28107
Reviewed-by: Jordi Vaquero <jordi.vaquero@metempsy.com>
Tested-by: kokoro <noreply+kokoro@google.com>