um: Don't mark stack executable

On one of my machines UML failed to start after enabling
SELinux.
UML failed to start because SELinux's execmod rule denies
executable pages on a modified file mapping.

Historically UML marks it's stack rwx.
AFAICT, these days this is no longer needed, so let's remove
PROT_EXEC.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>