[lld-macho] Ensure that chained fixups data comes first in __LINKEDIT

libstuff-based tools (e.g. `codesign` and `strip`) require
`__chainfixups` to be the first section in `__LINKEDIT`, and print a
"file not in an order that can be processed" error message if that is
not the case.

Differential Revision: https://reviews.llvm.org/D137492