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

bazel/emscripten_toolchain/BUILD.bazel

@@ -1,4 +1,4 @@
-load(":crosstool.bzl", "emscripten_cc_toolchain_config_rule")
+load(":toolchain.bzl", "emscripten_cc_toolchain_config_rule")
 
 package(default_visibility = ["//visibility:public"])
 
@@ -86,6 +86,13 @@ cc_toolchain_suite(
     },
 )
 
+toolchain(
+    name = "cc-toolchain-wasm",
+    target_compatible_with = ["@platforms//cpu:wasm32"],
+    toolchain = ":cc-compiler-wasm",
+    toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
+)
+
 py_binary(
     name = "wasm_binary",
     srcs = ["wasm_binary.py"],

bazel/emscripten_toolchain/toolchain.bzl

@@ -1105,7 +1105,7 @@ emscripten_cc_toolchain_config_rule = rule(
     attrs = {
         "cpu": attr.string(mandatory = True, values = ["asmjs", "wasm"]),
         "em_config": attr.label(mandatory = True, allow_single_file = True),
-        "emscripten_binaries": attr.label(mandatory = True),
+        "emscripten_binaries": attr.label(mandatory = True, cfg = "exec"),
         "script_extension": attr.string(mandatory = True, values = ["sh", "bat"]),
     },
     provides = [CcToolchainConfigInfo],

bazel/emscripten_toolchain/wasm_cc_binary.bzl

@@ -32,7 +32,7 @@ def _wasm_transition_impl(settings, attr):
         "//command_line_option:features": features,
         "//command_line_option:dynamic_mode": "off",
         "//command_line_option:linkopt": linkopts,
-        "//command_line_option:platforms": [],
+        "//command_line_option:platforms": ["@emsdk//:platform_wasm"],
         "//command_line_option:custom_malloc": "@emsdk//emscripten_toolchain:malloc",
     }