Add SDK items for linux and fix an issue with extra tools being shown on linux.

emsdk

@@ -643,7 +643,7 @@ class Tool:
     if WINDOWS and (hasattr(self, 'windows_url') or hasattr(self, 'windows_install_path')):
       return True
 
-    if LINUX or OSX and hasattr(self, 'unix_url'):
+    if (LINUX or OSX) and hasattr(self, 'unix_url'):
       return True
 
     return hasattr(self, 'url')

emsdk_manifest.json

@@ -452,6 +452,12 @@
     "uses": ["clang-incoming-64bit", "node-0.10.18-64bit", "emscripten-incoming"],
     "os": "osx"
   },
+  {
+    "version": "incoming",
+    "bitness": 64,
+    "uses": ["clang-incoming-64bit", "emscripten-incoming"],
+    "os": "linux"
+  },
   {
     "version": "master",
     "bitness": 32,
@@ -470,6 +476,12 @@
     "uses": ["clang-master-64bit", "node-0.10.18-64bit", "emscripten-master"],
     "os": "osx"
   },
+  {
+    "version": "master",
+    "bitness": 64,
+    "uses": ["clang-master-64bit", "emscripten-master"],
+    "os": "linux"
+  },
   {
     "version": "1.5.6",
     "bitness": 32,