wasm_cc_binary: Specify a default OS. Allow users to override platform.

Problem: proxy-wasm/proxy-wasm-cpp-sdk#157 (comment)

This is solving the problem in two different ways. Please let me know
your thoughts about both approaches, as either will work.

Signed-off-by: Martijn Stevenson <[email protected]>

Author: martijneken

bazel/BUILD

@@ -82,5 +82,6 @@ platform(
     name = "platform_wasm",
     constraint_values = [
         "@platforms//cpu:wasm32",
+        "@platforms//os:wasi",
     ],
 )

bazel/emscripten_toolchain/wasm_cc_binary.bzl

@@ -35,7 +35,7 @@ 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": ["@emsdk//:platform_wasm"],
+        "//command_line_option:platforms": [attr.platform],
         "//command_line_option:custom_malloc": "@emsdk//emscripten_toolchain:malloc",
     }
 
@@ -82,6 +82,9 @@ _WASM_BINARY_COMMON_ATTRS = {
     "exit_runtime": attr.bool(
         default = False,
     ),
+    "platform": attr.label(
+        default = "@emsdk//:platform_wasm",
+    ),
     "threads": attr.string(
         default = "_default",
         values = ["_default", "emscripten", "off"],