x86: compat_binfmt_elf Kconfig

This adds Kconfig and Makefile bits to build fs/compat_binfmt_elf.c,
just added.  Each arch that wants to use this file needs to add a
"select COMPAT_BINFMT_ELF" line in its Kconfig bits that enable COMPAT.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>