fix: JNI_OnLoad case

Subsurfer reported several issues related to JNI. Quick test exposed two issues:

Issue 1

JNI FindClass was not successful when was called from JNI_OnLoad (as result of System.loadLibrary for example) root case: top java class on the stack was java.lang.Runtime and its classLoader was NULL (corresponds to BootClassLoader). As result user class was looked in BC list and failed. fix: use systemClassLoader in case as top java class is BC class. issue is similar to robovm/robovm#352

Issue 2

If application uses a dynamic framework or library that exposes JNI_OnLoad it being called in context of application image. This happens as dyld loads all symbols from libraries referenced by LC_LOAD_DYLIB command. as result library JNI_OnLoad got called on early state while JNI is not initialized yet. This caused GPF due null pointer de-reference. fix: ignore JNI_OnLoad for application image

Fix was delivered as PR507.