x86: cleaned up vm.h (removed internal code)

This commit adds another include directory. Hopefully this doesn't
make things more confusing... Here's how includes work now:

moridin/kernel/

        inc/
                Header files that can be referenced by any other file.

        inc/arch
                Symbolic link to the archecture-specific include
                directory. e.g.
                inc/arch -> ../../arch/x86/inc

        arch/${ARCH}/inc
                Architecture-specific headers. This directory can be
                references from the kernel by taking advantage of the
                symbolic link explained above. E.g.

                #include <arch/vm.h>

                would actually include the file arch/${ARCH}/inc/vm.h.

        arch/${ARCH}/inc/${ARCH}
                This directory is for internal use for the architecture
                implementation. It should not be referenced from code
                outside arch/${ARCH}. Technically it could be referenced
                but it would required hardcoding the architecture, e.g.

                #include <arch/x86/paging.h>

                which is obviously architecture-specific.