Introduce expand_if_none_available to crosstool

This feature allows us to expand a flag_group when a build variable is not
available. This is helpful when migrating crosstools in a backward compatible
way (that works with released bazel as well as with bazel at HEAD).

RELNOTES: NONE.
PiperOrigin-RevId: 143955333
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 4e9722a..770d3ea 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
@@ -103,11 +103,13 @@
 
     repeated string expand_if_all_available = 4;
 
-    optional string expand_if_true = 5;
+    repeated string expand_if_none_available = 5;
 
-    optional string expand_if_false = 6;
+    optional string expand_if_true = 6;
 
-    optional VariableWithValue expand_if_equal = 7;
+    optional string expand_if_false = 7;
+
+    optional VariableWithValue expand_if_equal = 8;
   }
 
   message VariableWithValue {