add a wasm bazel toolchain (#603)

bazel/emscripten_toolchain/BUILD.bazel

@@ -0,0 +1,80 @@
+load(":crosstool.bzl", "emscripten_cc_toolchain_config_rule")
+
+package(default_visibility = ["//visibility:public"])
+
+filegroup(
+    name = "common-script-includes",
+    srcs = [
+        "emar.sh",
+        "emcc.sh",
+        "emscripten_config",
+        "env.sh",
+        "@emscripten//:all",
+        "@nodejs//:node_files",
+        "@npm//:node_modules",
+    ],
+)
+
+filegroup(
+    name = "compile-emscripten",
+    srcs = [":common-script-includes"],
+)
+
+filegroup(
+    name = "link-emscripten",
+    srcs = [
+        "emcc_link.sh",
+        "link_wrapper.py",
+        ":common-script-includes",
+        "@emscripten//:all",
+        "@nodejs//:node_files",
+    ],
+)
+
+filegroup(
+    name = "every-file",
+    srcs = [
+        ":compile-emscripten",
+        ":link-emscripten",
+        "@emscripten//:all",
+        "@nodejs//:node_files",
+    ],
+)
+
+filegroup(name = "empty")
+
+# dlmalloc.bc is implictly added by the emscripten toolchain
+cc_library(name = "malloc")
+
+emscripten_cc_toolchain_config_rule(
+    name = "wasm",
+    cpu = "wasm",
+    emscripten_version = "emscripten",
+)
+
+cc_toolchain(
+    name = "cc-compiler-wasm",
+    all_files = ":every-file",
+    ar_files = ":common-script-includes",
+    as_files = ":empty",
+    compiler_files = ":compile-emscripten",
+    dwp_files = ":empty",
+    linker_files = ":link-emscripten",
+    objcopy_files = ":empty",
+    strip_files = ":empty",
+    toolchain_config = "wasm",
+    toolchain_identifier = "emscripten-wasm",
+)
+
+cc_toolchain_suite(
+    name = "everything",
+    toolchains = {
+        "wasm": ":cc-compiler-wasm",
+        "wasm|emscripten": ":cc-compiler-wasm",
+    },
+)
+
+py_binary(
+    name = "wasm_binary",
+    srcs = ["wasm_binary.py"],
+)