target-ppc: Introduce DFP Helper Utilities

Add a new file (dfp_helper.c) to the PowerPC implementation for Decimal Floating
Point (DFP) emulation.  This first version of the file declares a structure that
will be used by DFP helpers.  It also implements utilities that will initialize
such a structure for either a long (64 bit) DFP instruction or an extended (128
bit, aka "quad") instruction.

Some utility functions are annotated with the unused attribute in order to preserve
build bisection.

Signed-off-by: Tom Musta <tommusta@gmail.com>
[agraf: Add never reached assert on dfp_prepare_rounding_mode()]
Signed-off-by: Alexander Graf <agraf@suse.de>
