lpython_floordiv causes linker error

[certik]

duplicate symbol '___lpython_overloaded_6___lpython_floordiv' in:
    CMakeFiles/matvec.dir/mat_vec-host-generated.c.o
    device/libmatvec.mod.a(mat_vec_device-generated.c.o)
ld: 1 duplicate symbol for architecture arm64

This is the same error as #2129, but now for a function from a different module.

[certik]

I have a very hackish workaround (I postprocess the generated C file with a Python script and remove the duplication...), so it's not a blocker, but this is a high priority.