[bazel] Switch to platforms-based toolchain resolution (#1036)

bazel/README.md

@@ -17,6 +17,9 @@ emsdk_deps()
 
 load("@emsdk//:emscripten_deps.bzl", emsdk_emscripten_deps = "emscripten_deps")
 emsdk_emscripten_deps(emscripten_version = "2.0.31")
+
+load("@emsdk//:toolchains.bzl", "register_emscripten_toolchains")
+register_emscripten_toolchains()
 ```
 The SHA1 hash in the above `strip_prefix` and `url` parameters correspond to the git revision of
 [emsdk 2.0.31](https://github.com/emscripten-core/emsdk/releases/tag/2.0.31). To get access to
@@ -26,8 +29,13 @@ parameter of `emsdk_emscripten_deps()`. Supported versions are listed in `revisi
 
 ## Building
 
-### Using wasm_cc_binary (preferred)
-First, write a new rule wrapping your `cc_binary`.
+Put the following line into your `.bazelrc`:
+
+```
+build --incompatible_enable_cc_toolchain_resolution
+```
+
+Then write a new rule wrapping your `cc_binary`.
 
 ```
 load("@rules_cc//cc:defs.bzl", "cc_binary")
@@ -54,17 +62,3 @@ and all of its dependencies, and does not require amending `.bazelrc`. This
 is the preferred way, since it also unpacks the resulting tarball.
 
 See `test_external/` for an example using [embind](https://emscripten.org/docs/porting/connecting_cpp_and_javascript/embind.html).
-
-### Using --config=wasm
-
-Put the following lines into your `.bazelrc`:
-```
-build:wasm --crosstool_top=@emsdk//emscripten_toolchain:everything
-build:wasm --cpu=wasm
-build:wasm --host_crosstool_top=@bazel_tools//tools/cpp:toolchain
-```
-
-Simply pass `--config=wasm` when building a normal `cc_binary`. The result of
-this build will be a tar archive containing any files produced by emscripten.
-See the [Bazel documentation](https://docs.bazel.build/versions/main/tutorial/cc-toolchain-config.html)
-for more details