Make CROSSTOOL.default_target_cpu optional

It's deprecated, and not used, there's no reason for it to be required.

RELNOTES: None.
PiperOrigin-RevId: 213639602

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 92eae89..3db28f2 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
@@ -520,7 +520,7 @@
   // Deprecated. Used to be the default CPU if not otherwise specified, but it's
   // not used for that purpose (or anything else) anymore. Please also don't add
   // new uses of this field.
-  required string default_target_cpu = 3;
+  optional string default_target_cpu = 3;
   // The default toolchain to use for each given cpu.
   repeated DefaultCpuToolchain default_toolchain = 4;