x86: processor.h - use PAGE_SIZE instead of numeric value

This patch replaces numeric constant with an appropriate macro

Also 0x800000000000UL is changed to bit shifting which is complement
to the code comment (thanks hpa for notice)

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>