swift234

[stdlib] Add default payload to Signaling NaN (#2494)

* [stdlib] Add default payload to Signaling NaN

Or it will have inifinity bit pattern.

* [stdlib] Skip isSignaling test on i386 arch

see: https://github.com/apple/swift/pull/2494

* [stdlib] Allow 0 payload for signaling NaN

Instead, use `_quietNaNMask >> 1` as the payload.
That is, we use only significandBitCount - 2 bits for NaN payload.