This corrects one case where floating-point exceptions could be set when
they're not supposed to be.
This also corrects a case where values were being treated as NaNs when
they weren't supposed to be.
This function is defined as always disabling the AHP bit in the fpcr
before performing any operations.
At the same time, rename the original FPUnpack function to FPUnpackBase
to match the pseudocode in the ARM reference manual.
