MC: Remove superfluous section attribute flag definitions

Summary:
llvm/MC/MCSectionMachO.h and llvm/Support/MachO.h both had the same
definitions for the section flags.  Instead, grab the definitions out of
support.

No functionality change.

Reviewers: grosbach, Bigcheese, rafael

Reviewed By: rafael

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2998