Add standalone_wasm feature to bazel emscripten_toolchain (#1145)

bazel/emscripten_toolchain/toolchain.bzl

@@ -441,6 +441,9 @@ def _impl(ctx):
             name = "output_format_js",
             enabled = True,
         ),
+        feature(
+            name = "wasm_standalone",
+        ),
     ]
 
     crosstool_default_flag_sets = [
@@ -1015,6 +1018,11 @@ def _impl(ctx):
             flags = ["-Werror"],
             features = ["wasm_warnings_as_errors"],
         ),
+        flag_set(
+            actions = all_link_actions,
+            flags = ["-sSTANDALONE_WASM"],
+            features = ["wasm_standalone"],
+        ),
     ]
 
     crosstool_default_env_sets = [

bazel/emscripten_toolchain/wasm_cc_binary.bzl

@@ -25,6 +25,9 @@ def _wasm_transition_impl(settings, attr):
     if attr.simd:
         features.append("wasm_simd")
 
+    if attr.standalone:
+        features.append("wasm_standalone")
+
     return {
         "//command_line_option:compiler": "emscripten",
         "//command_line_option:crosstool_top": "@emsdk//emscripten_toolchain:everything",
@@ -86,6 +89,9 @@ _WASM_BINARY_COMMON_ATTRS = {
     "simd": attr.bool(
         default = False,
     ),
+    "standalone": attr.bool(
+        default = False,
+    ),
     "_allowlist_function_transition": attr.label(
         default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
     ),