CGO constraint does nothing

Platform: Windows 11 x64

I cannot get cgo to work, and this is one of the ways I tried that is not being respected.

## Reproduction steps

Add the following to `BUILD.bazel`:

```
platform(
name = "x64_windows-clang-cl",
constraint_values = [
"@platforms//cpu:x86_64",
"@platforms//os:windows",
"@bazel_tools//tools/cpp:clang-cl",
"@rules_go//go/toolchain:cgo_on",
],
)
```

Run bazel with `--platforms=//tools:x64_windows-clang-cl --host_platform=//tools:x64_windows-clang-cl` and compile a cgo rule with `-s`.

## Expected behaviour

Either the rule is compiled with `CGO_ENABLED=1`, or an error occurs because the toolchain is not compatible with the `cgo_on` constraint.

## Actual behaviour

The rule is compiled with `CGO_ENABLED=0`

## What should happen

Either the `cgo_on` constraint should only be supported by toolchains that actually support cgo, or it should be removed.

Reproduce the issue using the `BUILD.bazel` platform definition, `--platforms=//tools:x64_windows-clang-cl`, `--host_platform=//tools:x64_windows-clang-cl`, and a cgo rule compiled with `-s`. Trace how the `@rules_go//go/toolchain:cgo_on` constraint is handled by the selected toolchain. Done means cgo is enabled, or the incompatible constraint produces an error, or the constraint is removed as requested.

Written by the indexing model from the issue text.