[piexwasm] Update README.md document emscripten details

 ◦ Emscripten has a core github these days:
  — update links to point to the core github and the .org docs
 ◦ Document which emscripten version built the piex-wasm release:
  — builder of last piex-wasm release was emscripten 1.38.27

Bug: 1132695
No-try: true
Change-Id: I703e1e7652ddebe98fb4328996fe6835f808f7e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2435128Reviewed-by: Austin Tankiang <austinct@chromium.org>
Commit-Queue: Austin Tankiang <austinct@chromium.org>
Auto-Submit: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811166}

ui/file_manager/image_loader/piex/README.md

-Install emscripten http://lmgtfy.com/?q=install+the+emscripten+sdk
+Install emscripten https://emscripten.org/docs/getting_started/index.html
 
 ```shell
-  git clone https://github.com/juj/emsdk.git
+  git clone https://github.com/emscripten-core/emsdk.git
   cd emsdk
   ./emsdk install latest
   ./emsdk activate latest
@@ -33,3 +33,5 @@ Release: submit piex.js.wasm piex.out.wasm to the Chromium repository
   git commit -a -m "Release piexwasm ..."
   git cl upload
 ```
+
+Release Builder: emscripten 1.38.27