debug: introduce debug_ll() and use it in abort()

the debug() console function is taking a lock before it access the console driver,
it does that by acquiring a mutex which may sleep.

since we want to be able to debug (and abort) in contexts where it's not possible sleep,
such as in page_fault, a lockless debug print method is introduced.

previousely to this patch, any abort on page_fault would cause an "endless" recursive
abort() loop which hanged the system in a peculiar state.