Provide "executable" in wasm_cc_binary's DefaultInfo. (#998)

This is needed to allow custom rules and/or transitions on top of wasm_cc_binary (e.g. to workaround #972).

It's debatable whether .wasm is the correct executable target for non-standalone Wasm builds, but we don't have a way to differentiate those in wasm_cc_binary, and considering that this attribute isn't exported now, it shouldn't break anything.

bazel/emscripten_toolchain/wasm_cc_binary.bzl

@@ -80,6 +80,7 @@ def _wasm_binary_impl(ctx):
     )
 
     return DefaultInfo(
+        executable = ctx.outputs.wasm,
         files = depset(outputs),
         # This is needed since rules like web_test usually have a data
         # dependency on this target.