techmap: add a Sklansky option for $lcu mapping

tests/techmap/sklansky.tcl

@@ -0,0 +1,15 @@
+yosys -import
+
+read_verilog +/choices/sklansky.v
+read_verilog -icells lcu_refined.v
+design -save init
+
+for {set i 1} {$i <= 16} {incr i} {
+    design -load init
+    chparam -set WIDTH $i
+    yosys proc
+    opt_clean
+    equiv_make lcu _80_lcu_sklansky equiv
+    equiv_simple equiv
+    equiv_status -assert equiv
+}