[mach-o] create __unwind_info section on x86_64

This is a minimally useful pass to construct the __unwind_info section in a
final object from the various __compact_unwind inputs. Currently it doesn't
produce any compressed pages, only works for x86_64 and will fail if any
function ends up without __compact_unwind.

rdar://problem/18208653

llvm-svn: 218703