Also change the device probing scheme to use something a bit more rational.
A current side-effect is that nubus cards are double-mapped. I expect
to fix that shortly.
Also change splclock() to block everything but serial hardware interrupts.
such as __warn_references() and __weak_reference() which are actually
machine dependant. This will make it easier for ports that are being
bootstraped with ELF and ECOFF based toolchains.
This change also introduces a new macro, _C_LABEL(x). _C_LABEL expands
its argument, an identifier, to a character string of the identifier
name as it is represented in an object file.
For most ports, _C_LABEL(x) will expand to "_x", for ELF based ports
_C_LABEL(x) will expand to "x".
