ld64.lld doesn't work with gcc, libtool or cctools' strip

libtool expects `ld64` to support `-r` and `-keep_private_externs`. gcc expects ld64 to support `-no_compact_unwind`. And cctools expect ld64 to support `-r` as well as to not require `-arch`.

All of these are missing in lld, preventing it to be a drop in replacement for ld64.

No files or tests are named; start by locating lld's ld64 option handling and the paths used by gcc, libtool, and cctools. Done means supporting -r, -keep_private_externs, and -no_compact_unwind, while allowing the cctools case without requiring -arch.

Written by the indexing model from the issue text.