arch/tile: avoid exporting a symbol no longer used by gcc

An earlier Tilera compiler generated calls to an "__ll_mul"
function for long long multiplication.  Our libgcc supported that
as an alias for the normal __muldi3 routine, so we made it available
to kernel modules as well.  However, for a while now the compiler
has internally been generating only the standard __muldi3 symbol,
and the version we are giving back to the community does not have
the __ll_mul alias, so we are removing it from the kernel too.

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