Revert "[ARM] Set default alignment to 64bits"

This reverts commit r367119.

This broke several bots:

http://lab.llvm.org:8011/builders/clang-atom-d525-fedora-rel/builds/26891/steps/ninja%20check%201/logs/FAIL%3A%20Clang%3A%3Aexception-alignment.cpp
http://green.lab.llvm.org/green/job/clang-stage1-cmake-RA-incremental/245/consoleFull

llvm-svn: 367166