moarvm: select libffi when building for MIPS

Otherwise the compilation will fail like this:

./libmoar.so: undefined reference to `dcCallbackThunkEntry'

See: https://github.com/MoarVM/MoarVM/issues/222



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>