Add support for invoke-static in optimizing compiler.

Support is limited to calls without parameters and returning
void. For simplicity, we currently follow the Quick ABI.

Change-Id: I54805161141b7eac5959f1cae0dc138dd0b2e8a5
diff --git a/compiler/utils/assembler.h b/compiler/utils/assembler.h
index cd4fc12..3dc5b5d 100644
--- a/compiler/utils/assembler.h
+++ b/compiler/utils/assembler.h
@@ -48,6 +48,23 @@
   class X86Assembler;
 }
 
+class ExternalLabel {
+ public:
+  ExternalLabel(const char* name, uword address)
+      : name_(name), address_(address) {
+    DCHECK(name != nullptr);
+  }
+
+  const char* name() const { return name_; }
+  uword address() const {
+    return address_;
+  }
+
+ private:
+  const char* name_;
+  const uword address_;
+};
+
 class Label {
  public:
   Label() : position_(0) {}