Remove references to specific seL4 headers.

There shouldn't be a need to use specific headers from libsel4. using
`sel4/sel4.h` will include all the relevant headers and should not cause
any conflicts that would also be compiler conflicts.

libsel4debug: Provide sel4_arch agnostic safe printf function

debug_safe_printf was previously specialized for ia32 due to seL4_GetIPCBuffer not
being implemented for it. This is not longer the case and the implementations can
be unified, as well as provided for other architectures that have existed since
this was originally written.