Use emsdk as external bazel dependency (#766)

* Makes provided bazel rules look up @emsdk workspace instead of local workspace
* Uses system-specific emscripten binaries instead of defaulting to linux
* Provides macros for loading emsdk dependencies (nodejs and emscripten binaries)
* Unhardcodes paths in bazel rules and .sh wrappers
* `update_bazel_workspace.sh` now updates `revisions.bzl`
* `emscripten_deps()` can be fed with specific emscripten version
* Adds external usage test

Addresses #650 and #696

test/test_bazel_mac.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+
+echo "test bazel"
+
+set -x
+set -e
+
+# Get the latest version number from emscripten-releases-tag.txt.
+VER=$(ggrep -oP '(?<=latest\": \")([\d\.]+)(?=\")' \
+        emscripten-releases-tags.txt \
+      | sed "s/\./\\\./g")
+# Based on the latest version number, get the commit hash for that version.
+HASH=$(grep "${VER}" emscripten-releases-tags.txt \
+      | grep -v latest \
+      | cut -f4 -d\")
+
+FAILMSG="!!! scripts/update_bazel_toolchain.sh needs to be run !!!"
+
+# Ensure the WORKSPACE file is up to date with the latest version.
+grep ${VER} bazel/revisions.bzl || (echo ${FAILMSG} && false)
+grep ${HASH} bazel/revisions.bzl || (echo ${FAILMSG} && false)
+
+cd bazel
+bazel build //hello-world:hello-world-wasm
+bazel build //hello-world:hello-world-wasm-simd
+
+cd test_external
+bazel build //:hello-world-wasm