[libomptarget][amdgpu][nfc] Replace use of gelf.h with libelf.h

AMDGPU can assume Elf64 so doesn't need to abstract over Elf32

Drop a few other unused headers at the same time. Now only llvm elf
and libelf are used by the plugin.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D106579