porting: use __builtin_ffsll() instead of ffsll()

- NetBSD/Darwin doesn't implement ffsll()
- use Compiler builtin

Change-Id: Iee78b4b81747b0bd3877fd2fcb98746f642ce080
BUG: 764655
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/8308
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Xavier Hernandez <xhernandez@datalab.es>
Reviewed-by: Justin Clift <justin@gluster.org>