wasm_cc_binary: Specify a default OS. Allow users to override platform. (#1262)

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

Problem: https://github.com/proxy-wasm/proxy-wasm-cpp-sdk/pull/157#discussion_r1277763118

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 <mstevenson@google.com>

* Rework platform selection to trigger os:wasi off standalone attr

Signed-off-by: Martijn Stevenson <mstevenson@google.com>

---------

Signed-off-by: Martijn Stevenson <mstevenson@google.com>

bazel/emscripten_toolchain/wasm_cc_binary.bzl

@@ -25,7 +25,9 @@ def _wasm_transition_impl(settings, attr):
     if attr.simd:
         features.append("wasm_simd")
 
+    platform = "@emsdk//:platform_wasm"
     if attr.standalone:
+        platform = "@emsdk//:platform_wasi"
         features.append("wasm_standalone")
 
     return {
@@ -35,7 +37,7 @@ 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": [platform],
         "//command_line_option:custom_malloc": "@emsdk//emscripten_toolchain:malloc",
     }