kexec-tools: Add support for build with x32 ABI in x86_64

Add autoconf test for detect when build is x32 ABI this enables to
test into purgatory Makefile to avoid use -mcmodel=large flag in CC.

Add ELFCLASS read and syscall number into kexec, see patch.

Signed-off-by: Aníbal Limón <anibal.limon@linux.intel.com>
Signed-off-by: Mariano Lopez <mariano.lopez@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>