Re-land #200 : waterfall fastcomp support (#204)

(This was reverted by mistake. It had a bad commit message though, so relanding with a nicer one is nice anyhow.)

With this, we can do emsdk install latest-fastcomp and it installs fastcomp from the waterfall. That is, we then have 3 main sdks people might want to use:

* latest which installs fastcomp-llvm (plus emscripten etc.) from the mozilla infrastructure. (fetches the last emscripten version there)
* latest-upstream which installs upstream-llvm (plus emscripten etc.) from the waterfall infrastructure. (fetches the last known good revision (lkgr) there)
* latest-fastcomp which installs fastcomp-llvm (plus emscripten etc.) from the waterfall infrastructure. (fetches the last known good revision (lkgr) there)

The first and last are currently somewhat overlapping in that both fetch a build of fastcomp. However, as we transition away from the mozilla infrastructure, we could just make latest an alias for latest-fastcomp. (And later, when we're ready to switch to the wasm backend by default, the alias could switch to latest-upstream.)

emsdk_manifest.json

@@ -199,6 +199,15 @@
     "activated_path": "%installation_dir%/emscripten",
     "activated_cfg": "LLVM_ROOT='%installation_dir%/bin';BINARYEN_ROOT='%installation_dir%'"
   },
+  {
+    "id": "waterfall",
+    "version": "fastcomp-%waterfall-lkgr%",
+    "bitness": 64,
+    "linux_url": "https://storage.googleapis.com/wasm-llvm/builds/linux/%waterfall-lkgr%/wasm-binaries.tbz2",
+    "install_path": "fastcomp/%waterfall-lkgr%",
+    "activated_path": "%installation_dir%/emscripten",
+    "activated_cfg": "LLVM_ROOT='%installation_dir%/fastcomp/bin';BINARYEN_ROOT='%installation_dir%'"
+  },
   {
     "id": "clang",
     "version": "e1.13.0",
@@ -1549,6 +1558,12 @@
     "uses": ["waterfall-upstream-%waterfall-lkgr%-64bit", "node-8.9.1-64bit"],
     "os": "linux"
   },
+  {
+    "version": "fastcomp-%waterfall-lkgr%",
+    "bitness": 64,
+    "uses": ["waterfall-fastcomp-%waterfall-lkgr%-64bit", "node-8.9.1-64bit"],
+    "os": "linux"
+  },
   {
     "version": "%precompiled_tag32%",
     "bitness": 32,