Fix MSVC clz/ctz impl

[zcyemi]

The previous implementation of ctz32 incorrectly used _BitScanForward,
which can cause division by zero exception(Crash) in js_atod when using like var x = 3.0.

Details:
in js_atod

radix_shift = ctz32(radix);
radix1 = radix >> radix_shift;

ctz32 return wrong value making radix1 to be 0, which propagates to mul_pow and causes l = (f + d - 1) / d with d = 0.
Then crash.