x86: unify TASK_ALIGN definitions

This patch moves the TASK_ALIGN constraints to common header.
The base of it is the same for x86_64 and i386. The only difference
is the presence of vSMP in x86_64. As it's not a worry in i386, we can
safely use the same code for both.

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>