[libunwind] Fix executable stack directive on Linux.
[libunwind] Fix executable stack directive on Linux.

Summary:
Disable executable stack on Linux. Also remove redundant Android check
as it is covered by Android.

Reviewers: phosek, compnerd, rs, rmaprath, EricWF, krytarowski

Reviewed By: krytarowski

Subscribers: srhines, llvm-commits, krytarowski

Differential Revision: https://reviews.llvm.org/D33217

git-svn-id: 62ef012455b6b8fcec9c41ca270ebf409aae1006@303206 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libunwind,llvm-mirror/libunwind,llvm-mirror/libunwind