RFR: 8358339: Handle MethodCounters::_method backlinks after JDK-8355003

[Aleksey Shipilev]

Found this when reading mainline-vs-premain webrev. [JDK-8355003](https://bugs.openjdk.org/browse/JDK-8355003) introduced a backlink to `Method*` in `MethodCounters`. I believe we need to handle that backlink at least in `CodeBuffer::finalize_oop_references()`. premain does this, while mainline does not. Also, amusingly, we have `MethodCounters::is_methodCounters`, but not the super-class `Metadata::is_methodCounters`. 

I pulled in the hunks that use `is_methodCounters()` and `MethodCounters::method()` from premain into this PR.

Additional testing:
 - [x] Linux x86_64 server fastdebug, `runtime/cds`
 - [ ] Linux x86_64 server fastdebug, `tier1`
 - [ ] Linux x86_64 server fastdebug, `all`

-------------

Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/25599/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25599&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358339
  Stats: 10 lines in 3 files changed: 10 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25599.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25599/head:pull/25599

PR: https://git.openjdk.org/jdk/pull/25599