Use the core IRT when running sel_ldr tests.

We don't ship the chrome IRT with the NaCl SDK anymore.

BUG=https://code.google.com/p/naclports/issues/detail?id=60

Review URL: https://codereview.chromium.org/11645017

git-svn-id: http://naclports.googlecode.com/svn/trunk/src@651 7dad1e8b-422e-d2af-fbf5-8013b78bd812