kernel/x86_64: rewrite cpuid.S -> cpuid.cpp

Just getting rid of some assembly, no functional change.

Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
kernel/x86_64: implement x86_swap_pgdir in C++

No reason not to inline this function.

Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
kernel/x86_64: use C++11 style static assert

Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>