Update links for AIX and Linux on Z downloads (#940)

Now that binaries are being built as part of release:

- Update the links for AIX and Linux on Z on the download
  page so that they point to the binaries available on
  the community download site instead of the
  external IBM download page.
- Add AIX, Linux s390x and Linux PPC BE to the helper
  used to generate release announcements.

layouts/partials/secondary-download-matrix.hbs

@@ -25,11 +25,11 @@
 
       <tr>
         <th>Linux on System z</th>
-        <td colspan="6"><a href="http://www.ibm.com/developerworks/web/nodesdk/">Download (Unofficial, provided by IBM)</a></td>
+        <td colspan="6"><a href="https://nodejs.org/dist/{{version.node}}/node-{{version.node}}-linux-s390x.tar.xz">64-bit</a></td>
       </tr>
       <tr>
         <th>AIX on Power Systems</th>
-        <td colspan="6"><a href="http://www.ibm.com/developerworks/web/nodesdk/">Download (Unofficial, provided by IBM)</a></td>
+        <td colspan="6"><a href="https://nodejs.org/dist/{{version.node}}/node-{{version.node}}-aix-ppc64.tar.gz">64-bit</a></td>
       </tr>
     </tbody>
   </table>

scripts/helpers/downloads.js

@@ -39,6 +39,18 @@ const postMergeDownloads = [
     'title': 'Linux PPC LE 64-bit Binary',
     'templateUrl': 'https://nodejs.org/dist/v%version%/node-v%version%-linux-ppc64le.tar.xz'
   },
+  {
+    'title': 'Linux PPC BE 64-bit Binary',
+    'templateUrl': 'https://nodejs.org/dist/v%version%/node-v%version%-linux-ppc64.tar.xz'
+  },
+  {
+    'title': 'Linux s390x 64-bit Binary',
+    'templateUrl': 'https://nodejs.org/dist/v%version%/node-v%version%-linux-s390x.tar.xz'
+  },
+  {
+    'title': 'AIX 64-bit Binary',
+    'templateUrl': 'https://nodejs.org/dist/v%version%/node-v%version%-aix-ppc64.tar.gz'
+  },
   {
     'title': 'SunOS 32-bit Binary',
     'templateUrl': 'https://nodejs.org/dist/v%version%/node-v%version%-sunos-x86.tar.xz'