libdfp: Move common code to generic place
libdfp: Move soft-dfp code to soft-dfp folder
PowerPC: fix copysignd128
PowerPC: fix issignaling

This patch fixes the wrong DCM constant used where it identifies 0.0
as being a signaling NaN.
libdfp: Add check-abi test [WIP]
libdfp: Fix quantize test FP constants
libdfp: Fix missing __dpd_trunctdsd2 um software implementation
libdfp: Add local PLT calls check