pvh: Add ELF Note definitions

Define ELF Note header structures and necessary for parsing
the PVH entry point address encoded in the kernel ELF header.

Generated the elf.rs file again by running bindgen:

bindgen --with-derive-default elf.h > elf.rs

From upstream linux include/uapi/linux/elf.h at commit:
3cc6e2c599cdca573a8f347aea5da4c855ff5a78
and then edited to eliminate unnecessary definitions, add comments,
and relocate definitions and tests for clarity.

Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>

Author: aljimenezb