RFR: 8301244: Replace legacy pragma implementations in HotSpot

[Julian Waters]

HotSpot has a few legacy implementations of pragmas in some of its macros, they should be cleaned up and instead use a shared new PRAGMA macro available to all platforms

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Commit messages:
 - compilerWarnings_gcc.hpp
 - compilerWarnings_visCPP.hpp
 - Introduce PRAGMA

Changes: https://git.openjdk.org/jdk/pull/12255/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12255&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301244
  Stats: 10 lines in 3 files changed: 4 ins; 2 del; 4 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