build: use wxneeded on openbsd

On OpenBSD 6.0 and greater W^X is enabled by default. All executables
that violate W^X need to be marked with PT_OPENBSD_WXNEEDED. In
addition to this, they must be executed from a filesystem mounted with
'wxallowed'.

More info on W^X: https://en.wikipedia.org/wiki/W%5EX

PR-URL: https://github.com/nodejs/node/pull/9232
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

common.gypi

@@ -276,6 +276,9 @@
           ['_type=="static_library" and OS=="solaris"', {
             'standalone_static_library': 1,
           }],
+          ['OS=="openbsd"', {
+            'ldflags': [ '-Wl,-z,wxneeded' ],
+          }],
         ],
         'conditions': [
           [ 'target_arch=="ia32"', {