[lld] Fixes for unitests in standalone builds (#83670)

1. Replace the obsolete `llvm_add_unittests()` CMake function with an
explicit check for `TARGET llvm_gtest`. This is more consistent with the
rest of the code, and it makes it possible to avoid checking out
`third-party` tree.
2. Add `LLDUnitTests` test dependency to standalone builds. It is
defined unconditionally, and actually necessary to ensure that unit
tests will be built.