+ WebAssembly threads: set IsMultiThread to true in WasiBeginThread

rtl/wasi/systhrd.inc

@@ -411,6 +411,7 @@ Var
 
 begin
   {$IFDEF DEBUGWASMTHREADS}DebugWriteln('WasiBeginThread(sa: '+IntToStr(PtrUint(Sa))+',ss: '+IntToStr(PtrUint(StackSize))+',TF: '+IntToStr(PtrUint(ThreadFunction))+',Arg: '+IntToStr(PtrUint(P))+',fl: '+IntToStr(PtrUint(CreationFlags))+',ID: '+IntToStr(PtrUint(ThreadID))+')');{$ENDIF}
+  IsMultiThread:=true;
   New(T);
   T^.StackBlock:=nil;
   T^.TLSBlock:=nil;