Blackfin: clean up trace buffer handling when crashing

Blackfin: clean up trace buffer handling when crashing

Avoid banging on the trace MMRs when debugging is disabled, avoid calling
the funcs multiple times in a row, disable the trace buffer earlier in the
exception handler to avoid eating more user entries, and dump the buffer
before calling the kgdb hook.  This way we maximize useful debugging info
up front rather than needing external tools (like gdb/serial/etc...).

Signed-off-by: Mike Frysinger <vapier@gentoo.org>