Framework target: fixing back JNI support

Previous rework broke JNI. Rework added logic to start JVM automatically once framework is loaded to provide ObjectiveC classes from Java side available as soon as possible. A bad thing is that JVM structures required for JNI call were not exposed. A try to initialize(second) JVM with JNI_CreateJavaVM will fail.
Goal of the fix is to provide ability to have both JNI and ObjectiveC framework operational.

Disabling automatic JVM startup

This can be controlled with another key to robovm.xml. Instead, automatic JVM startup will be disabled if JNI_CreateJavaVM is present in the list of exported symbols:

<exportedSymbols>
    <symbol>JNI_CreateJavaVM</symbol>
</exportedSymbols>

Compiler will generate(only in case of Framework target) _bcFrameworkSkipJavaVMStartup symbol that will be recognized by native code and JVM start up will be skipped.

Interoperability

Once JVM is created it is a good idea(but subject for JNI code design) to ping back framework support code to pre-load all objective-c custom classes. In this case JNI based code should call following function:

void rvmInitializeFrameworkWithJVM(JavaVM* externalVm, JNIEnv *externalEnv);

Code

Code was delivered as PR497