[mach-o] make __unwind_info defer to __eh_frame when necessary.

Not all situations are representable in the compressed __unwind_info format,
and when this happens the entry needs to point to the more general __eh_frame
description.

Just x86_64 implementation for now.

rdar://problem/18208653

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@219836 91177308-0d34-0410-b5e6-96231b3b80d8