[jdk21u] RFR: 8312591: GCC 6 build failure after JDK-8280982

[Aleksey Shipilev]

Clean backport to upgrade older toolchains.
Risk is low, adds a few warning exclusions.

Additional testing:
 - [ ] GHA

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Commit messages:
 - Backport 9454b2bbe130fdbe86485b928b80d19156c709ee

Changes: https://git.openjdk.org/jdk21u/pull/32/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=32&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312591
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u/pull/32.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/32/head:pull/32

PR: https://git.openjdk.org/jdk21u/pull/32