WIP support for setting the iostd

Author: pepijndevos

himbaechel/uarch/gowin/gowin.h

@@ -135,6 +135,7 @@ NPNR_PACKED_STRUCT(struct Constraint_POD {
     int32_t row;
     int32_t col;
     int32_t bel;
+    int32_t iostd;
 });
 
 NPNR_PACKED_STRUCT(struct Extra_package_data_POD {

himbaechel/uarch/gowin/gowin_arch_gen.py

@@ -251,11 +251,12 @@ class PackageExtraData(BBAStruct):
 
     def serialise_lists(self, context: str, bba: BBAWriter):
         bba.label(f"{context}_constraints")
-        for (net, row, col, bel) in self.cst:
+        for (net, row, col, bel, iostd) in self.cst:
             bba.u32(self.strs.id(net).index)
             bba.u32(row)
             bba.u32(col)
             bba.u32(ord(bel[0])-ord('A')+IOBA_Z)
+            bba.u32(self.strs.id(iostd).index if iostd else 0)
 
     def serialise(self, context: str, bba: BBAWriter):
         bba.slice(f"{context}_constraints", len(self.cst))