Fix executable stack directive on Linux.

Summary: Use __linux__  to check for Linux and bring back the check for __GNU__.

Reviewers: echristo, krytarowski, compnerd, rengolin

Reviewed By: krytarowski

Subscribers: phosek, llvm-commits, srhines

Differential Revision: https://reviews.llvm.org/D33219

llvm-svn: 303131