Example:
```
DEBUG: /tmp/winsdk/51c9aa8f783c7ef42fa88749bcc55eec/external/bazel_tools/tools/cpp/lib_cc_configure.bzl:118:5: 
Auto-Configuration Warning: registry query result for VC 8.0:                                                  

STDOUT(start)

STDOUT(end)
STDERR(start):
java.io.IOException: Cannot run program "C:\Windows\system32\reg.exe" (in directory "/tmp/winsdk/51c9aa8f783c7ef42fa88749bcc55eec/external/local_config_winsdk"): error=2, No such file or directory                           
STDERR(end)
```

The errors are not fatal though. Blocking https://github.com/bazelbuild/bazel/issues/8572 Not a release blocker: the affected file is not embedded into Bazel.
Windows: winsdk_configure errors on other platforms
### Description of the problem / feature request:

Bazel workspace's `winsdk_configure()` repository rule runs (and throws java.io.IOExceptions) on every platform, not just Windows.

### Bugs: what's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

1. get https://github.com/bazelbuild/bazel/commit/47d0173cc92f5544c0cca718a1f393a01fc863e9
2. `bazel build src:bazel`
3. `export CC_CONFIGURE_DEBUG=1`
4. `bazel-bin/src/bazel build --nobuild //src:bazel`

### What operating system are you running Bazel on?

Linux

### If `bazel info release` returns "development version" or "(@non-git)", tell us how you built Bazel.

built at https://github.com/bazelbuild/bazel/commit/47d0173cc92f5544c0cca718a1f393a01fc863e9

src/main/res/winsdk_configure.bzl
src/main/res/winsdk_configure.bzl
diff --git a/src/main/res/winsdk_configure.bzl b/src/main/res/winsdk_configure.bzl
index 2eb7d0e2ef838e..3b77acbc8a537b 100644
--- a/src/main/res/winsdk_configure.bzl
+++ b/src/main/res/winsdk_configure.bzl
@@ -14,6 +14,16 @@
 
 """Configure local Resource Compilers for every available target platform.
 
+Platform support:
+
+- Linux/macOS/non-Windows: this repository rule is almost a no-op.
+  It generates an empty BUILD file, and does not actually discover any
+  toolchains. The register_local_rc_exe_toolchains() method is a no-op.
+
+- Windows: this repository rule discovers the Windows SDK path, the installed
+  rc.exe compilers. The register_local_rc_exe_toolchains() registers a toolchain
+  for each rc.exe compiler.
+
 Usage:
 
     load("//src/main/res:winsdk_configure.bzl", "winsdk_configure")
@@ -46,6 +56,9 @@ def _find_rc_exes(root):
     return result
 
 def _find_all_rc_exe(repository_ctx):
+    if not repository_ctx.os.name.startswith("windows"):
+        return {}
+
     vc = find_vc_path(repository_ctx)
     if vc:
         env = setup_vc_env_vars(
