Fix clang build issues.

Also add PREAMBLE macro to allow head of interpreter loop to be simply hoisted.

Change-Id: Id4a3f81d56b5981e1a497cf2bc72f1c263364fe5
diff --git a/src/class_linker.cc b/src/class_linker.cc
index 08b26e4..6029c83 100644
--- a/src/class_linker.cc
+++ b/src/class_linker.cc
@@ -74,7 +74,7 @@
 
 namespace art {
 
-extern "C" JValue artInterpreterToQuickEntry(Thread* self, ShadowFrame* shadow_frame);
+JValue artInterpreterToQuickEntry(Thread* self, ShadowFrame* shadow_frame);
 
 static void ThrowNoClassDefFoundError(const char* fmt, ...)
     __attribute__((__format__(__printf__, 1, 2)))