x86: vdso ELF handling - use SELFMAG instead of numeric constant

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: akpm@linux-foundation.org
Cc: hpa@zytor.com
Cc: mingo@elte.hu
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>