boot/x86_64: call debug_cleanup() before preparing mmu data

long_mmu_init() prepares initial paging structures for 64 bit kernel.
Once that function completes bootloader cannot allocate any memory
that needs to be passed to the kernel.

Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
boot/x86_64: convert address of previous_debug_output

Together with the previous commit should fix #10535.

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