Generalization of libc+c

Hello everyone,

We are looking in to adding the ability to remotely run compiler-rt tests on build targets.
Something similar to how it is done for libc++ tests.
The first approach and the discussion could be find in ⚙ D83429 [libc++] [test] Introduce the `--test-executable` option in

Now I would like to generalize libcxx/utils/ and move it to llvm/utils/

I will propose patches shortly, unless someone thinks that generalizing is not a good idea.