Celltype `$alu` is marked evaluable but misses eval implementation

### Version

Yosys 0.32+76 (git sha1 73cb4977b2e, clang++ 11.1.0 -fPIC -Os)

### On which OS did this happen?

macOS

### Reproduction Steps

One symptom is that running `sim` on a design containing an `$alu` cell will lead to abort in `CellTypes::eval`.

### Expected Behavior

No aborts

### Actual Behavior

Aborts