resolve a fixme: the "nonexecutable stack directive" is actually

resolve a fixme: the "nonexecutable stack directive" is actually
a .section.  Switch to it with SwitchSection.

However, I think that this directive should be safe on any ELF target.
If so, we should hoist it up out of the X86 and SystemZ targets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@94298 91177308-0d34-0410-b5e6-96231b3b80d8