RFR: 8361055: Serial: Inline SerialHeap::process_roots

[Albert Mingkun Yang]

Refactor roots processing in Serial (young-gc and full-gc) to clean up the control-flow and make is clearer what roots and closures are used in each context.

Test: tier1-8

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

Commit messages:
 - sgc-roots

Changes: https://git.openjdk.org/jdk/pull/26038/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26038&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361055
  Stats: 84 lines in 4 files changed: 12 ins; 51 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/26038.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26038/head:pull/26038

PR: https://git.openjdk.org/jdk/pull/26038