RFR: 8310537: Fix -Wconversion warnings in gcUtil.hpp

[Coleen Phillimore]

Please review small changes to fix -Wconversion warnings in gc code.  See CR for details.
Tested with tier1 on Oracle platforms.

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Commit messages:
 - simple fix

Changes: https://git.openjdk.org/jdk/pull/14585/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14585&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310537
  Stats: 20 lines in 5 files changed: 0 ins; 0 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/14585.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14585/head:pull/14585

PR: https://git.openjdk.org/jdk/pull/14585