Large performance regression on Windows due to indirect branch link breakage

Commit 638fb106 PR #4886 accidentally broke Windows indirect branch table updating. This was discovered by a user: https://groups.google.com/d/msgid/dynamorio-users/CAO1ikSY7Uvgcw_b98H1ifkvFGhDUSYzZe9uatrYNPcETMYix-Q%40mail.gmail.com?utm_medium=email&utm_source=footer

This is pretty bad: this will cause massive slowdowns on some programs. We need to put out an updated 9.1 release.

Performance regression tests are not always simple to set up. Maybe we can more easily have a test that checks the exit stats.