Add AsmPrinter support for emitting a directive to declare that

Add AsmPrinter support for emitting a directive to declare that
the code being generated does not require an executable stack.

Also, add target-specific code to make use of this on Linux
on x86.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@50634 91177308-0d34-0410-b5e6-96231b3b80d8