x86: thread_info: merge thread_info allocation

Make them similar so that both use THREAD_ORDER and THREAD_FLAGS and have a
THREAD_SIZE definition that is setup in asm/page_xx.h

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>