Add the grep tool. This is only needed if one wants to run the 'other.test_llvm_list' test for LLVM itself, Emscripten doesn't care much about this.

emsdk_manifest.json

@@ -749,6 +749,12 @@
     "activated_cfg": "CRUNCH='%installation_dir%/crunch'",
     "activated_path": "%installation_dir%"
   },
+  {
+    "id": "gnu", // Would really want to name this 'grep' but llvm-lit has a bug that it fails to find 'grep' tool if it is located in a directory that has the word 'grep' as a substring.
+    "version": "2.5.4",
+    "windows_url": "grep-2.5.4.zip",
+    "activated_path": "%installation_dir%/bin"
+  },
   {
     "id": "mingw",
     "version": "4.6.2",