Dispatches calls to JNIHelpers to the appropriate architecture-specific class.
This is necessary until the PowerPC JNI design is changed to match the design
for IA32 JNI (i.e. until PowerPC uses C code to do var args passing on both sides).
When PPC uses C var arg passing, the code can be refactored and moved from the JNI
helpers into a single, architecture-independent class.