Refactor handling of -std and -stdlib in toolchain (#6601)

This introduces the first pieces of a cleaner way to configure toolchain
components on target dimensions: dedicated features for those target
dimensions.

With that, we extract a `libcxx_feature` that can always be present but
disables its flags on unsupported targets.

With `-stdlib` in its own feature, move `-std=c++20` to not require
a variable but directly live in the flags.

This should enable us to extract the largest remaining feature into its
own file cleanly by removing dynamic configuration of it, along with
libcxx.

Further refactoring of target-specific logic will follow in its
footsteps.

MODULE.bazel.lock

@@ -252,7 +252,7 @@
   "moduleExtensions": {
     "//bazel/cc_toolchains:clang_configuration.bzl%clang_toolchain_extension": {
       "general": {
-        "bzlTransitiveDigest": "bboGyUY23/USpijt9AwiA8C2A4TI3wom/e7F3b5LeCI=",
+        "bzlTransitiveDigest": "ryYQP3j7NfvoUF+l/TDK6eiWjEfXgeBQSVVL+iS6hwc=",
         "usagesDigest": "lTxkeAFhR0iBEa3dg5hWvtd2HFCr5zCJx/fl27A+IKA=",
         "recordedFileInputs": {},
         "recordedDirentsInputs": {},