kernel/x86_64: try to inline physical page mapper functions

The way we handle paging is very wasteful and relies heavily on virtual
funcions even if there is absolutely no reason to do so. The proper
solution would be to do a major rework of paging code (including
arch-independent parts).

On x86_64 physical page mapper is very simple what makes the overhead
resulting from the desing of paging interface very expensive. This
patch attempts to make things a bit better by helping GCC with
devirtualization and allowing inlining physical page mapper impementation
(well, only when it is devirtualized).
kernel/x86_64: use C++11 atomics in X86PagingMethod64Bit