MEDIUM: debug: do not dump a context-less backtrace in ABORT_NOW()

MEDIUM: debug: do not dump a context-less backtrace in ABORT_NOW()

Originally before 2.4, ABORT_NOW() was used to instantly stop the program
with as little damage as possible in order to help debug it, keeping
registers intact.

This was modified in 2.4-dev6 by commit 5baf4fe31ad ("MEDIUM: debug:
now always print a backtrace on CRASH_NOW() and friends") because the
same macro was shared with BUG_ON() and we didn't have the backtrace.
But by doing this we lost the ability to debug the precise crash
location.

Later in 3.0, we added support for an extra contextual info with commit
d417863828 ("MINOR: debug: support passing an optional message in
ABORT_NOW()"). This became even more fishy because at this point,
ABORT_NOW() called empty would only emit a backtrace, while when
passed an argument, it would emit "ABORT at <file>:<line>: <arg>",
losing the registers despite what the comment would say.

Now that we can type call places, it becomes possible to fix it again
so that ABORT_NOW() honors its two promises:
  - no register mangling when called with no argument ;
  - location + backtrace + message when called with an argument.

This patch simply conditions the call to ha_backtrace_to_stderr() to
the presence of an argument, without touching the abort_with_line()
call. It's done using an extra if to minimize the diff as it's only
temporary. For BUG_ON(), the trace calls are factored before the call
to an argument-less ABORT_NOW() since that one will not print the
message anymore.