RFR: 8288048: Build failure with GCC 6 after JDK-8286562

[Aleksey Shipilev]

[JDK-8286562](https://bugs.openjdk.org/browse/JDK-8286562) added the `-Wstringop-overflow` pragma, which cannot be understood by GCC 6. There are already ways to make such pragmas conditional on GCC version.

Additional testing:
 - [x] Linux x86_64 `fastdebug` build with GCC 6
 - [x] Linux x86_64 `fastdebug` build with GCC 9

@YaSuenag, if you could test with GCC 12, that would be helpful.

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

Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/9090/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9090&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288048
  Stats: 11 lines in 3 files changed: 10 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9090.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9090/head:pull/9090

PR: https://git.openjdk.java.net/jdk/pull/9090