Allow setting supports_embedded_runtimes crosstool capability using f…

…eature

`supports_embedded_runtimes` can now be expressed using 'static_link_cpp_runtimes' feature (should be enabled for it to take effect).

This cl allows toolchain owners to express that toolchain supports embedded runtimes in the same way as other crosstool capabilities are expressed.

This cl is a step towards bazelbuild/bazel#5883. Also
see the rollout doc here:
https://docs.google.com/document/d/1uv4c1zag6KvdI31qdx8C6jiTognXPQrxgsUpVefm9fM/edit#.

Flag removing legacy behavior is bazelbuild/bazel#6861

RELNOTES: None.
PiperOrigin-RevId: 227024509

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

@@ -356,6 +356,7 @@ message CToolchain {
   optional bool supports_start_end_lib = 28 [default = false];
   // Legacy field, use 'supports_interface_shared_libraries' instead.
   optional bool supports_interface_shared_objects = 32 [default = false];
+  // Legacy field, use 'static_link_cpp_runtimes' feature instead.
   optional bool supports_embedded_runtimes = 40 [default = false];
   // If specified, Blaze finds statically linked / dynamically linked runtime
   // libraries in the declared crosstool filegroup. Otherwise, Blaze