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

bazel/test_external/BUILD

@@ -0,0 +1,12 @@
+load("@emsdk//emscripten_toolchain:wasm_rules.bzl", "wasm_cc_binary")
+
+cc_binary(
+    name = "hello-world",
+    srcs = ["hello-world.cc"],
+)
+
+wasm_cc_binary(
+    name = "hello-world-wasm",
+    cc_target = ":hello-world",
+)
+