RFR: 8327049: Stop exporting debug.cpp functions on gcc [v2]

[Magnus Ihse Bursie]

> This is a partial backout of [JDK-8326509](https://bugs.openjdk.org/browse/JDK-8326509). It turned out that starting to export the functions from debug.cpp could have unintended consequences in certain circumstances, so revert back to the original situation where these functions were only exported on Windows.

Magnus Ihse Bursie has updated the pull request incrementally with one additional commit since the last revision:

  Use WINDOWS_ONLY instead

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18062/files
  - new: https://git.openjdk.org/jdk/pull/18062/files/04cacdfa..3323265e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18062&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18062&range=00-01

  Stats: 5 lines in 1 file changed: 0 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18062.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18062/head:pull/18062

PR: https://git.openjdk.org/jdk/pull/18062