[cxx] use antlir distro toolchain when building for distro platform

Summary:
The previous diff makes a CXX toolchain that uses antlir2 images to build cxx
targets that link against an upstream centos platform, rather than the fbcode
platform.

This diff automatically selects this toolchain when the platform constraint is
active.

Test Plan:
```
❯ buck2 test fbcode//antlir/distro/toolchain/cxx/tests:
Buck UI: https://www.internalfb.com/buck2/bf305863-ab98-4c05-923d-40450a119488
Test UI: https://www.internalfb.com/intern/testinfra/testrun/2251800074356858
Tests finished: Pass 8. Fail 0. Fatal 0. Skip 0. Build failure 0
```

```
❯ buck2 test -c fbcode.arch=aarch64 fbcode//antlir/distro/toolchain/cxx/tests:
Buck UI: https://www.internalfb.com/buck2/d79f7370-2e29-411a-ac64-53789f28b9da
Test UI: https://www.internalfb.com/intern/testinfra/testrun/17169973639622886
Tests finished: Pass 8. Fail 0. Fatal 0. Skip 0. Build failure 0
```

Reviewed By: dtolnay

Differential Revision: D66986700

fbshipit-source-id: d96336154a0c9e5924a8a446e30e805aa4b4241f

antlir/distro/toolchain/cxx/tests/BUCK

@@ -3,25 +3,45 @@ load("//antlir/antlir2/bzl/feature:defs.bzl", "feature")
 load("//antlir/antlir2/bzl/image:defs.bzl", "image")
 load("//antlir/antlir2/bzl/package:defs.bzl", "package")
 load("//antlir/antlir2/testing:image_test.bzl", "image_python_test")
-load("//antlir/bzl:build_defs.bzl", "cpp_library")
+load("//antlir/bzl:build_defs.bzl", "cpp_binary", "cpp_library")
 load("//antlir/distro/platform:defs.bzl", "alias_for_current_image_platform", "default_image_platform")
 
 oncall("antlir")
 
 prelude = native
 
-prelude.cxx_binary(
+cpp_binary(
     # @autodeps-skip
     name = "main",
     srcs = ["main.cpp"],
-    _cxx_toolchain = "//antlir/distro/toolchain/cxx:toolchain",
     # This should only be built for a centos system platform, use compatibility
     # attrs to guarantee that
     compatible_with = [
         "//antlir/distro:build-for-distro",
     ],
     default_target_platform = default_image_platform("centos9"),
-    platform_preprocessor_flags = [
+    deps = [
+        ":dep",
+        ":platform-preprocessor-flags",
+        "//antlir/distro/deps/jsoncpp:jsoncpp",
+        "//antlir/distro/deps/libgcc:stdc++",
+    ],
+)
+
+cpp_library(
+    # @autodeps-skip
+    name = "dep",
+    srcs = ["dep.cpp"],
+    headers = ["dep.h"],
+    default_target_platform = default_image_platform("centos9"),
+    deps = [
+        "//antlir/distro/deps/rpm:librpm",
+    ],
+)
+
+prelude.cxx_library(
+    name = "platform-preprocessor-flags",
+    exported_platform_preprocessor_flags = [
         (
             "centos9-x86_64$",
             ['-DPLATFORM_PREPROCESSOR_FLAG="centos9-x86_64"'],
@@ -39,23 +59,6 @@ prelude.cxx_binary(
             ['-DPLATFORM_PREPROCESSOR_FLAG="centos10-aarch64"'],
         ),
     ],
-    deps = [
-        ":dep",
-        "//antlir/distro/deps/jsoncpp:jsoncpp",
-        "//antlir/distro/deps/libgcc:stdc++",
-    ],
-)
-
-cpp_library(
-    # @autodeps-skip
-    name = "dep",
-    srcs = ["dep.cpp"],
-    headers = ["dep.h"],
-    _cxx_toolchain = "//antlir/distro/toolchain/cxx:toolchain",
-    default_target_platform = default_image_platform("centos9"),
-    deps = [
-        "//antlir/distro/deps/rpm:librpm",
-    ],
 )
 
 alias_for_current_image_platform(