Set stack size to 4 MiB for the second JNAerator pass

Many -libraryOverrides overrides consume lots of stack space, and
can easily lead to StackOverflowErrors during JNAeration. The
smallest stack size that seems to work is around 2 MiB, but it is
set to 4 MiB to leave some headroom for different versions of GLib
that may require more overrides.