libdfp: decContextSymbols names cleanup

This patch remove the symbol redefinition header decContextSymbols.h
from libdecnumber and adjust the functions that calls its functions.
libdfp: libdecnumber symbol name cleanup

This patch remove more uneeded symbol redefinition headers.  More
specifically decCommonSymbols.h, decDPDSymbols.h, decDoubleSymbols.h,
decPackedSymbols.h, decQuadSymbols.h, and decSingleSymbols.h.
libdfp: Use libdecnumber compare on ilogbd

This patch changes the way to find which rounding to use by using
libdecnumber compare instruction instead of the compiler generate
compare instructions.
libdecnumber: Move common definition under DPD

This patch moves headers with common decimal defintions (range,
exponent, etc.) to libdecnumber common folder.  This is to avoid the
need to duplicate it on bid folder and also to organize headers better.
Fix
Fix
libdecnumber: Move common debug directive to common header

This patch moves the implementation of decNumberShow to a common header.
libdecnumber: Add initial BID support for _Decimal64

This patch add Binary Integer Decimal encoding support on libdecnumber
for _Decimal64 type.  All libdfp tests now pass, with some exceptions
due ULPs issues (since x86_64 does not have a ULP tests file).