Allow for INT widths different from FP widths

Allows e.g. an FP16-only FPU to support INT8 and INT32:

localparam fpu_features_t CUSTOM = '{
    Width:         32,
    EnableVectors: 1'b0,
    EnableNanBox:  1'b1,
    FpFmtMask:     5'b00100,
    IntFmtMask:    4'b1110
  };