Replace some LLVM-specific code with something more general.

This basically gives any compiler a place to hang extra private data
off art::Compiler, even though only LLVM needs it right now.

Change-Id: I408778ea1010ab2ad9ec4810e5c53d468a8772c2
diff --git a/src/compiler.cc b/src/compiler.cc
index 8e8958b..fe4f4fc 100644
--- a/src/compiler.cc
+++ b/src/compiler.cc
@@ -282,11 +282,9 @@
       support_debugging_(support_debugging),
       stats_(new AOTCompilationStats),
       image_classes_(image_classes),
-#if defined(ART_USE_LLVM_COMPILER)
-      compiler_llvm_(NULL),
-#endif
       compiler_library_(NULL),
       compiler_(NULL),
+      compiler_context_(NULL),
       jni_compiler_(NULL),
       create_invoke_stub_(NULL)
 {