[Libomptarget] Replace use of `dlopen` with LLVM's dynamic library support

This patch replaces uses of `dlopen` and `dlsym` with LLVM's support
with `loadPermanentLibrary` and `getSymbolAddress`. This allows us to
remove the explicit dependency on the `dl` libraries in the CMake. This
removes another explicit dependency and solves an issue encountered
while building on Windows platforms. The one downside to this is that
the LLVM library does not currently support `dlclose` functionality, but
this could be added in the future.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D131507