[lldb] Fix flakyness in TestGdbRemote_vContThreads

The cause is the non-async-signal-safety printf function (et al.). If
the test managed to interrupt the process and inject a signal before the
printf("@started") call returned (but after it has actually written the
output), that string could end up being printed twice (presumably,
because the function did not manage the clear the userspace buffer, and
so the print call in the signal handler would print it once again).

This patch fixes the issue by replacing the printf call in the signal
handler with a sprintf+write combo, which should not suffer from that
problem (though I wouldn't go as far as to call it async signal safe).