RFR: 8282209: G1: Use synchronous concurrent marking for the code cache

[Leo Korinth]

This fix removes collect_for_codecache(), and uses collect() directly instead. This has the advantage that the g1 code path can be run concurrently using a concurrent mark instead of a full gc.

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Commit messages:
 - 8282209: G1: Use synchronous concurrent marking for the code cache

Changes: https://git.openjdk.java.net/loom/pull/102/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=102&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282209
  Stats: 132 lines in 11 files changed: 5 ins; 106 del; 21 mod
  Patch: https://git.openjdk.java.net/loom/pull/102.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/102/head:pull/102

PR: https://git.openjdk.java.net/loom/pull/102