[tests] Add a hack to eliminate some dangling .s files on buildbots.

 - Benjamin fixed the emission of this file in r179937, but it still lives on a
   few buildbots. We should probably clean up the build dirs once in a while,
   eh?

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188527 91177308-0d34-0410-b5e6-96231b3b80d8