lld/mach-o: Infer darwinnew from filename ld64.lld.darwinnew too

`-flavor` is difficult to use through the clang driver since it
must be the first argument.

clang's `-fuse-ld=foo` looks for `ld64.foo` when targeting darwin,
so it's easiest if darwinnew accepts some `ld64.foo`. Let's go with
`ld64.lld.darwinnew`, so that `clang -fuse-ld=lld.darwinnew` does
the right thing (assuming a symlink with the name `ld64.ld.darwinnew
exists in the right place).

This is temporary until darwinnew replaces ld64.lld, and it only
exists to make testing the new lld port easier.