windows: enable watching signals with process.on('SIGXYZ')

This reverts commit ea1cba6246.

The offending commit was intended to land on the v0.8 branch only, but
it accidentally got merged at some point.

Closes #5054.

src/node.js

@@ -733,10 +733,6 @@
   };
 
   startup.processSignalHandlers = function() {
-    // Not supported on Windows.
-    if (process.platform === 'win32')
-      return;
-
     // Load events module in order to access prototype elements on process like
     // process.addListener.
     var signalWraps = {};