[APFloat] Implement PPCDoubleDouble add and subtract.

Summary:
I looked at libgcc's implementation (which is based on the paper,
Software for Doubled-Precision Floating-Point Computations", by Seppo Linnainmaa,
ACM TOMS vol 7 no 3, September 1981, pages 272-283.) and made it generic to
arbitrary IEEE floats.

Differential Revision: https://reviews.llvm.org/D26817

llvm-svn: 289472