lib: tevent: make TEVENT_SIG_INCREMENT atomic.

lib: tevent: make TEVENT_SIG_INCREMENT atomic.

On arm platforms incrementing a variable is not
an atomic operation, so may be interrupted by
signal processing (if a signal interrupts another
signal handler).

Use compiler built-ins to make this atomic.
__sync_fetch_and_add() works on gcc, llvm,
IBM xlC on AIX, and Intel icc (10.1 and
above).

atomic_add_32() works on Oracle Solaris.

Based on an inital patch from kamei@osstech.co.jp.

Bug #10640 - smbd is not responding - tevent_common_signal_handler() increments non-atomic variables

https://bugzilla.samba.org/show_bug.cgi?id=10640

Back-ported from master 536c799f00d7bdd6a574b6bdbc0e9c742eeef8b5

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <Volker.Lendecke@SerNet.DE>