[lld-macho] Switch default to new Darwin backend

The new Darwin backend for LLD is now able to link reasonably large
real-world programs on x86_64. For instance, we have achieved
self-hosting for the X86_64 target, where all LLD tests pass when
building lld with itself on macOS. As such, we would like to make it the
default back-end.

The new port is now named `ld64.lld`, and the old port remains
accessible as `ld64.lld.darwinold`

This [annoucement email][1] has some context. (But note that, unlike
what the email says, we are no longer doing this as part of the LLVM 12
branch cut -- instead we will go into LLVM 13.)

Numerous mechanical test changes were required to make this change; in
the interest of creating something that's reviewable on Phabricator,
I've split out the boring changes into a separate diff (D95905). I plan to
merge its contents with those in this diff before landing.

(@gkm made the original draft of this diff, and he has agreed to let me
take over.)

[1]: https://lists.llvm.org/pipermail/llvm-dev/2021-January/147665.html

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D95204

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