undef JNIEXPORT in hello.m

This avoids a build error with recent versions of OpenJDK.
disable runtime code generation in OpenJDK reflection implementation

By default, OpenJDK's reflection implementation will generate bytecode
at runtime for certain operations.  This doesn't play well with iOS,
where JIT compilation is not allowed.  Fortunately, there's a system
property which controls this behavior, and by setting it to
Integer.MAX_VALUE we can effectively disable it.