Support the GNU non-exec stack annotation system

Support the GNU non-exec stack annotation system
On recent GNU ELF systems one can mark an object file as not
requiring an executable stack. If all objects- linked into a
program have this note then the program will not use an executable
stack, which is good for security (and some distros have it as a
QA policy). GHC generated code does not need an executable stack
so add the note to the assembly output of the native code
generator (conditional on a configure test).