fix Method.getModifiers crash due to bootimage miscompile

When calculating field offsets in the bootimage generator, we failed
to consider alignment at inheritence boundaries (i.e. the last field
inherited by from a superclass should be followed by enough padding to
align the first non-inherited field at a machine word boundary).  This
led to a mismatch between native code and Java code in terms of class
layouts, including that of java.lang.reflect.Method.