[Dexter] Remove the Windows dependency on lld from CMake

Currently, lld is marked as a dependency on Windows in
cross-project-tests/CMakeLists.txt which means CMake will fail if lld isn't
enabled. The idea of the cross-project-tests is that tests that don't have
their dependencies met should just be unsupported.

Remove the depenency from the CMake step and check whether Dexter's
platform-specific dependencies have been met in
cross-project-tests/lit.cfg.py. If the dependencies are met then add 'dexter'
to the available_features, otherwise don't and the dexter tests will be
"UNSUPPORTED".

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D115872