Do not use CROSSTOOL to select cc_toolchain

This is a preparation for flipping
--incompatible_disable_cc_toolchain_label_from_crosstool_proto, which will stop
reading default_toolchain fields from the crosstool and will rely on
cc_toolchain_suite.toolchains containing entries for cpu without compiler.

Migrations docs can be found here:
https://docs.bazel.build/versions/master/skylark/backward-compatibility.html#disallow-using-crosstool-to-select-the-cc_toolchain-label.

RELNOTES: None.
PiperOrigin-RevId: 216470358

third_party/com/github/bazelbuild/bazel/src/main/protobuf/crosstool_config.proto

diff --git a/third_party/com/github/bazelbuild/bazel/src/main/protobuf/crosstool_config.proto b/third_party/com/github/bazelbuild/bazel/src/main/protobuf/crosstool_config.proto
index 3db28f2..5afc52c 100644
--- a/third_party/com/github/bazelbuild/bazel/src/main/protobuf/crosstool_config.proto
+++ b/third_party/com/github/bazelbuild/bazel/src/main/protobuf/crosstool_config.proto
@@ -521,7 +521,8 @@
   // not used for that purpose (or anything else) anymore. Please also don't add
   // new uses of this field.
   optional string default_target_cpu = 3;
-  // The default toolchain to use for each given cpu.
+  // Deprecated: The default toolchain to use for each given cpu.
+  // Use cc_toolchain_suite.toolchains with only cpu specified instead
   repeated DefaultCpuToolchain default_toolchain = 4;
 
   // All the toolchains in this release.