The significandl() function shall return the mantissa of x, sig such that x ≡ sig × 2n scaled such that 1 ≤ sig < 2.
Note: This function is intended for testing conformance to IEC 60559/IEEE 754 Floating Point, and its use is not otherwise recommended.
This function is equivalent to scalb(x, (double)-ilogb(x)).
Upon successful completion, significandl() shall return the mantissa of x in the range 1 ≤ sig < 2.
If x is 0, ħHUGE_VALL, or NaN, the result is undefined.