Windows tool paths should have `.exe` suffix

This is somewhat of a cross-post of https://github.com/bazel-contrib/rules_foreign_cc/issues/1391. On Windows, omitting the `.exe` extension can work in some scenarios (direct invocation from shell), but fail in others.

Most notably, CMake will issue the confusing error of being able to query the GCC version (it can call the binary), but then reporting that no compiler exists at the path given.

When resolving MSYS2/MinGW/MSVC toolchains on Windows, we should ensure that the full path is given.

I already had to introduce this in an unrelated PR: https://github.com/bazelbuild/rules_cc/pull/390/commits/e5be0933b5984533a19f4405220983e7d6db4057