add a wasm bazel toolchain (#603)

bazel/hello-world/BUILD

@@ -0,0 +1,12 @@
+load("@rules_cc//cc:defs.bzl", "cc_binary")
+load("//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",
+)