Use configuration flags for //src/conditions:darwin_arm64

Partially reverts 6d637f4, to allow
cross-compiling Bazel from darwin_x86_64 to darwin_arm64 with
the `--cpu=darwin_arm64` flag.

The only source that is different between darwin_x86_64 and darwin_arm64
now is the embedded JDK.

Closes bazelbuild#12653.

PiperOrigin-RevId: 346298051

src/conditions/BUILD

@@ -1,3 +1,5 @@
+load("@bazel_skylib//lib:selects.bzl", "selects")
+
 filegroup(
     name = "srcs",
     srcs = glob(["**"]),
@@ -89,11 +91,28 @@ config_setting(
 )
 
 config_setting(
-    name = "darwin_arm64",
+    name = "darwin_arm64_constraint",
     constraint_values = [
         "@platforms//os:macos",
         "@platforms//cpu:arm64",
     ],
+)
+
+config_setting(
+    name = "darwin_arm64_flag",
+    values = {"cpu": "darwin_arm64"},
+)
+
+# Workaround for an issue where the dummy cc toolchain isn't being picked up
+# when cross-compile from darwin_x86_64 to darwin_arm64 cpu.
+# TODO(https://github.com/bazelbuild/bazel/issues/12655): Remove the flag based
+# select when the issue is resolved.
+selects.config_setting_group(
+    name = "darwin_arm64",
+    match_any = [
+        ":darwin_arm64_constraint",
+        ":darwin_arm64_flag",
+    ],
     visibility = ["//visibility:public"],
 )