core: adjust System.Runtime.Platform interop

Part of #1055.

Added `Platform` method to compiler.

pkg/compiler/syscall.go

@@ -68,6 +68,7 @@ var syscalls = map[string]map[string]Syscall{
 		"CheckWitness":           {"System.Runtime.CheckWitness", false},
 		"Log":                    {"System.Runtime.Log", false},
 		"Notify":                 {"System.Runtime.Notify", false},
+		"Platform":               {"System.Runtime.Platform", false},
 	},
 	"storage": {
 		"ConvertContextToReadOnly": {"System.Storage.AsReadOnly", false},

pkg/interop/runtime/runtime.go

@@ -80,3 +80,9 @@ func GetNotifications(h []byte) [][]interface{} {
 func GetInvocationCounter() int {
 	return 0
 }
+
+// Platform returns the platform name, which is set to be `NEO`. This function uses
+// `System.Runtime.Platform` syscall.
+func Platform() []byte {
+	return nil
+}