Made generated prettyasserts more debuggable

The main star of the show is the adoption of __builtin_trap() for
aborting on assert failure. I discovered this GCC/Clang extension
recently and it integrates much, _much_ better with GDB.

With stdlib's abort(), GDB drops you off in several layers of internal
stdlib functions, which is a pain to navigate out of to get to where the
assert actually happened. With __builtin_trap(), GDB stops immediately,
making debugging quick and easy.

This is great! The pain of debugging needs to come from understanding
the error, not just getting to it.

---

Also tweaked a few things with the internal print functions to make
reading the generated source easier, though I realize this is a rare
thing to do.

scripts/bench.py

@@ -1366,13 +1366,6 @@ def run(runner, bench_ids=[], **args):
                 '-ex', 'break %s:%d' % (path, lineno),
                 '-ex', 'run',
                 '--args']
-        elif failure.assert_ is not None:
-            cmd[:0] = args['gdb_path'] + [
-                '-q',
-                '-ex', 'run',
-                '-ex', 'frame function raise',
-                '-ex', 'up 2',
-                '--args']
         else:
             cmd[:0] = args['gdb_path'] + [
                 '-q',