[Libomptarget] Replace libelf with LLVM's Elf libraries

This patch replaces the dependency on `libelf` with LLVM's ELF support.
With this patch the user no-longer needs to have `libelf` on their
system to build and configure OpenMP offloading. The replacement is
mostly mechanical, with the exception of the hash table support which
was added in D131309.

Depends on D131309

Reviewed By: JonChesterfield, saiislam

Differential Revision: https://reviews.llvm.org/D131401