Account for signed multiply
Account for signed multiply

When multiply is done in native words, the conversion to words from the
vp_vector4_t vectors must be done signed. This only matters if the
input operands are different sizes (and themselves signed) but will
not hurt even if we want an  unsigned result.
CastMi/iverilog,CastMi/iverilog,themperek/iverilog,themperek/iverilog,themperek/iverilog,CastMi/iverilog