Allow setting supports_fission crosstool capability using feature

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

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: 226950450

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

@@ -368,6 +368,7 @@ message CToolchain {
   optional bool supports_incremental_linker = 41 [default = false];
   // Legacy field, ignored by Bazel.
   optional bool supports_normalizing_ar = 26 [default = false];
+  // Legacy field, use 'per_object_debug_info' feature instead.
   optional bool supports_fission = 43 [default = false];
   // Legacy field, ignored by Bazel.
   optional bool supports_dsym = 51 [default = false];