RFR: Generation sizing fixes

[William Kemper]

Two small fixes:
* Fix windows build
* Need gc id for resumed old generation marking

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Commit messages:
 - Fix MAX2 arguments
 - Use GCIdMark when resuming old marking

Changes: https://git.openjdk.org/shenandoah/pull/182/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=182&range=00
  Stats: 2 lines in 2 files changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/182.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/182/head:pull/182

PR: https://git.openjdk.org/shenandoah/pull/182