[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

llvm-svn: 303206