MINOR: compiler: rely on builtin detection for __builtin_unreachable()

MINOR: compiler: rely on builtin detection for __builtin_unreachable()

Due to __builtin_unreachable() only being associated to gcc 4.5 and
above, it turns out it was not enabled for clang. It's not used *that*
much but still a little bit, so let's enable it now. This reduces the
code size by 0.2% and makes it a bit more efficient.

(cherry picked from commit 41fc18b1d1a37e2a6645ea4a8fb4d2d5f009fc0c)
[wt: backported to shut a gcc warning in debug.c with USE_THREAD=0]
Signed-off-by: Willy Tarreau <w@1wt.eu>