MINOR: debug: make a pair of __BUG_ON() for modern and obsolete linkers

MINOR: debug: make a pair of __BUG_ON() for modern and obsolete linkers

The _BUG_ON() and _BUG_ON_ONCE() calls used to systematically call
__DBG_COUNT() first, then to call __BUG_ON(). Let's have a variant of
__BUG_ON() for each linker mode, that is merged with __DBG_COUNT().
The modern linker one now performs both the counting and processing
of the event at once.

For now the "once" aspect is still dependent on the expression-local
counter so that the code is exactly the same as before. The code size
didn't change.