freebsd,linux,sunos: make stack non-executable

Link with -z,noexecstack to make stack memory non-executable.  Makes
shellcode injection through buffer overflows more difficult.

Fixes: https://github.com/joyent/node/issues/7542
PR-URL: https://github.com/node-forward/node/pull/8
Reviewed-By: Trevor Norris <trevnorris@gmail.com>