Update bit manipulator functions to use built-ins when supported

[r30shah]

In an offline discussion with @Spencer-Comin, we found that some of the built-ins for example for leadingZeros, trailingZeros and popcnt were used on few platforms but not all. We found that GCC on Linux on Z also had built-ins which can be used. I am opening up this issue to keep track of adding usage of such built-ins on supported platforms.