[jdk11u-dev] RFR: 8335801: [11u] Backport of 8210988 to 11u removes gcc warnings

[Andrew John Hughes]

The backport of 8210988 contained a number of issues, the most serious of which is it completely removed warnings from `gcc` builds by changing `WARNING_CFLAGS_JDK` and `WARNING_CFLAGS_JVM`, but not defining `WARNINGS_ENABLE_ALL`, `WARNINGS_ENABLE_ADDITIONAL_JVM` and `DISABLED_WARNINGS`.

The other more minor issues are the silent inclusion of `-Wreorder` to clang builds (should be a backport of [JDK-8208672](https://bugs.openjdk.org/browse/JDK-8208672)) and `-xc99=%none` (reverting [JDK-8215296](https://bugs.openjdk.org/browse/JDK-8215296)), because new lines were used as is without comparing differences in what was being replaced.

All three of these changes show up clearly when comparing the 11u & 12u changesets, and are corrected by this patch.

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Commit messages:
 - 8335801: [11u] Backport of 8210988 to 11u removes gcc warnings

Changes: https://git.openjdk.org/jdk11u-dev/pull/2838/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2838&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335801
  Stats: 8 lines in 1 file changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2838.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2838/head:pull/2838

PR: https://git.openjdk.org/jdk11u-dev/pull/2838