[lld-macho][nfc] Decouple tapi-link.s test from libSystem

If we fix https://github.com/llvm/llvm-project/issues/54184, we will end
up including libSystem in every %lld invocation, which would break
tapi-link.s as it assumes that libSystem isn't directly linked (instead
it goes through libReexportSystem).

Let's remove this unnecessary coupling, as well as use `split-file`
instead of having a separate file under `Inputs`.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D120939