MINOR: debug: print gdb hints when crashing

MINOR: debug: print gdb hints when crashing

To make bug reporting easier for users, when crashing, let's suggest
what to do. Typically when a BUG_ON() matches, only the current thread
is useful the vast majority of the time, while when the watchdog
triggers, all threads are interesting.

The messages are printed at the end after the dump. We may adjust these
with wiki links in the future is more detailed instructions are relevant.

(cherry picked from commit 8f204fa8aeadef3faea4471ba9cfd93d9d168960)
Signed-off-by: Willy Tarreau <w@1wt.eu>