Toplevel scopes were only getting defaulted to private visibility
for the startup interpreter, which does not ever use in-IR call
protocol. When the toplevel script body is first optimized to
fullint or compiled to bytecode, call protocol is installed with
no external preamble code to set visibility to private, so the
visibility was defaulting back to public as in non-script scopes.
This change adds visibility to the frame push instruction, so top-
level scripts will use private in their frame push.

This fixes regression in test:jruby:aot test_timeout.rb toplevel
timeout test, which relied on the non-self call to timeout to see
the toplevel private method and trigger method_missing. This began
to fail with my timeout update because the method is now defined
in Ruby code rather than manually in Java code (the latter of
which explicitly set private visibility).