RFR: 8301244: Replace legacy pragma implementations in HotSpot's compilerWarnings.hpp [v8]

[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 four additional commits since the last revision:

 - gcc
 - VISCPP
 - Comment
 - Re-add PRAGMA

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12255/files
  - new: https://git.openjdk.org/jdk/pull/12255/files/de049888..8ba0ff35

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12255&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12255&range=06-07

  Stats: 11 lines in 3 files changed: 3 ins; 2 del; 6 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