Provide a floorl() function (which is currently only able to call GCC's
__builtin_floorl() function) for systems which don't provide one.

Change-Id: Ie0140ff195f0eae525c7bd70c7c3e23039fde569
Reviewed-on: https://code.wireshark.org/review/5889
Reviewed-by: Stephen Fisher <sfisher@sdf.org>