[libc][NFC] Remove dependecies on entrypoints from loader tests.

A small collection of utils called osutils has been added. These utils
are now used in loader tests instead of the equivalent entrypoints.
The loader can now be built and tested without depending on any entrypoint.
In a follow up change, the relevent entrypoints will be updated to use the
newly added osutils.

Reviewed By: lntue, michaelrj

Differential Revision: https://reviews.llvm.org/D115655