Math library code has quite a few places with complex bit
logic that are ultimately fed into a copysign. This helps
avoid some regressions in a future patch.
This assumes the position in the float type, which should
at least be valid for IEEE types. Not sure if we need to guard
against ppc_fp128 or anything else weird.
There appears to be some value in simplifying the value operand
as well, but I'll address that separately.
