RFR: 8346278: Clean up some flag handing in flags-cflags.m4

[Magnus Ihse Bursie]

WARNING_CFLAGS_JVM should not be used to set arbitrary warning flags for Hotspot.

We do not need to check for gcc 6+ anymore.

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Commit messages:
 - 8346278: Clean up some flag handing in flags-cflags.m4

Changes: https://git.openjdk.org/jdk/pull/22762/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22762&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346278
  Stats: 20 lines in 2 files changed: 0 ins; 15 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/22762.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22762/head:pull/22762

PR: https://git.openjdk.org/jdk/pull/22762