set InvocationTargetException target

When using the OpenJDK classpath, the target exception and the
Throwable cause are two different fields; we must set them both.
ignore instance variable for static method invocations
fix sign extension from constant to register on x86

scalac may generate a ldc followed by an l2i, whereas javac always
seems to condense this into a single ldc_w.  The former exposed a bug
in the JIT compiler which we never hit with javac-generated bytecode.