Move arrayCopy / identityHashCode from OpenJdkJvm to System.

These functions are only ever called as the JNI implementation of
System.arrayCopy (and never directly from native code) so it's
cleaner to keep them as part of System's native implementation.

Moreover, this change fixes issues in debug mode where taking the
address of a function in a different shared library (potentially
compiled with different compilation flags) generates incorrect
code.

Change-Id: I0da147b9917eb1bb418d3f253fb2bfad46e53e0c