According to http://www.opensource.apple.com/source/cctools/cctools-782/RelNotes/Private_CompilerTools.html, this release:
Added support for the Intel AES instructions. Radar bug
Earlier we were using a very old cctools. Currently, we are using cctools 758, which is perhaps therefore also lacking Intel AES support. Support for these instructions are vital to get good performance, so we should perhaps consider upgradring to cctools 782 or newer, and check if this works.
cctools was upgraded to 782 in r30007 as part of bug 5257.