[libomptarget][amdgpu] don't declare Elf_Note on FreeBSD

On FreeBSD, the system `<libelf.h>` already declares `struct Elf_Note`
indirectly (via `<sys/elf_common.h>`). This results in compile errors
when building the libomptarget amdgpu plugin. Avoid redeclaring `struct
Elf_Note` on FreeBSD to fix the errors.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D107661