Enable SafeStack for CloudABI.

Summary:
I've got a patchset in my home directory to integrate support for
SafeStack into CloudABI's C library. All of the CloudABI unit tests
still seem to pass. Pretty sweet!

This change adds the necessary changes to Clang to make
-fsanitize=safe-stack work on CloudABI. Without it, passing this command
line flag throws an error.

Reviewers: eugenis, samsonov

Differential Revision: http://reviews.llvm.org/D17243

llvm-svn: 261135