libdfp: Add quantumd[32|64|128] software implementation

The quantumdN functions compute the quantum of a finite argument.
If x is infinite, the result is +infinity. If x is NaN, the result
is NaN.
PowerPC: optimized quantumd[32|64|128] implementation
libdfp: Add llquantexpd[32|64|128] software implementation

The llquantexpdN functions compute the quantum exponent of a finite argument.
If x is infinite or NaN, they compute LLONG_MIN and a domain error occurs.
PowerPC: optimized llquantexpd[32|64|128]
libdfp: Fix test-logb

Fix the test-logb with correct expected results. Also fix scaffold
macro to expected for results with variations.