tile: support gcc 7 optimization to use __multi3

The tip gcc includes an optimization mode that converts
64-bit divides into 128-bit multiplies using __multi3.
Export the symbol so that modules can find it.  We just
export unconditionally without worrying about the gcc
version, since the symbol has been in libgcc forever and
the function is less than 300 bytes.

Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>