criu/files-reg.c: add build-id validation functionality

efi.h: Required for accessing the build-id of .efi files

This adds functions to find, store and compare with the stored build-id.
get_build_id() calls 32-bit or 64-bit helper functions depending on
the bitness of the ELF file after first ensuring that it is actually
an ELF file by checking for the magic number.

The number of iterations while searching the elf file for the
build-id before giving up (500 while searching the note section)
are limited.

Signed-off-by: Ajay Bharadwaj <ajayrbharadwaj@gmail.com>