softfloat: Revert and reimplement remaining parts of b645bb4885 and 5a6932d51d

softfloat: Revert and reimplement remaining parts of b645bb4885 and 5a6932d51d

Revert the parts of commits b645bb4885 and 5a6932d51d which are still
in the codebase and under a SoftFloat-2b license.

Reimplement support for architectures where the most significant bit
in the mantissa is 1 for a signaling NaN rather than a quiet NaN,
by adding handling for SNAN_BIT_IS_ONE being set to the functions
which test values for NaN-ness.

This includes restoring the bugfixes lost in the reversion where
some of the float*_is_quiet_nan() functions were returning true
for both signaling and quiet NaNs.

[This is a mechanical squashing together of two separate "revert"
and "reimplement" patches.]

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1421073508-23909-4-git-send-email-peter.maydell@linaro.org