Add hybrid assembler concept to ARM assembler.

src/IceFixups.h

Index: src/IceFixups.h
diff --git a/src/IceFixups.h b/src/IceFixups.h
index 43284ac9fefb8b3f81fc40c35ec0e6d7c1eb3e0f..c84633fdc099edafa7eb6b51aefd288ef2fb5e32 100644
--- a/src/IceFixups.h
+++ b/src/IceFixups.h
@@ -45,7 +45,9 @@ public:
 
   void set_value(const Constant *Value) { value_ = Value; }
 
-  void emit(GlobalContext *Ctx, RelocOffsetT OverrideOffset) const;
+  // Emits fixup, then returns the number of bytes to skip.
+  virtual size_t emit(GlobalContext *Ctx, RelocOffsetT OverrideOffset,
+                      bool IsPCRel) const;
 
 private:
   intptr_t position_ = 0;