ensure debug helper functions are retained by linker

Apple's linker tends to remove functions which are never called, which
is not what we want for e.g. vmPrintTrace, since that function is only
intended to be called interactively from within GDB.

src/machine.cpp

@@ -4402,7 +4402,7 @@ noop()
 } // namespace vm
 
 // for debugging
-void
+JNIEXPORT void
 vmPrintTrace(Thread* t)
 {
   class Visitor: public Processor::StackVisitor {
@@ -4444,7 +4444,7 @@ vmPrintTrace(Thread* t)
 }
 
 // also for debugging
-void*
+JNIEXPORT void*
 vmAddressFromLine(Thread* t, object m, unsigned line)
 {
   object code = methodCode(t, m);