test: Make gccgo believe that the variables can change.

This test is supposed to test runtime behaviour.  However,
gccgo is smart enough to see that the variables are local to
the package and that they never change.  Add a function so
that gccgo thinks that the variables can in fact vary, so that
the test works at runtime as expected.

R=rsc
CC=golang-dev
https://golang.org/cl/2178041