RFR: 8301244: Replace legacy pragma implementations in HotSpot's compilerWarnings.hpp [v7]

[Julian Waters]

> HotSpot has a few legacy implementations of pragmas in some of its macros in compilerWarnings.hpp, they should be cleaned up and use a more modern approach. Rename PRAGMA macro in compilerWarnings_gcc.hpp as well since it can be used for more than just compiler warnings

Julian Waters has updated the pull request incrementally with one additional commit since the last revision:

  compilerWarnings_gcc.hpp

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12255/files
  - new: https://git.openjdk.org/jdk/pull/12255/files/25984854..de049888

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12255&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12255&range=05-06

  Stats: 4 lines in 1 file changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/12255.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12255/head:pull/12255

PR: https://git.openjdk.org/jdk/pull/12255