Revert "Use the integrated assembler by default on 32-bit PowerPC and SPARC"

This reverts commit r225212.  It's failing on multiple buildbots [1][2].

[1]: http://lab.llvm.org:8011/builders/clang-x86_64-debian-fast/builds/22032
[2]: http://lab.llvm.org:8080/green/view/Clang/job/clang-stage1-cmake-RA-incremental_check/2357/

llvm-svn: 225221