Make libglvnd work with SELinux

Created by: kbrenneman

The generated entrypoints stubs in libGLdispatch and libGLX depend on being able to allocate pages that are both writable and executable. That might not be allowed on systems that use SELinux.

The normal way around that is to create an anonymous file and map it twice, once for reading and once for writing: http://www.akkadia.org/drepper/selinux-mem.html