RFR: 8304288: Add foreign linker tag set descriptions

[Jorn Vernee]

Add foreign linker tag set descriptions for `foreign+downcall`, `foreign+upcall`, which are printed in -Xlog:help message.

Sample output:


Described tag sets:
 logging: Logging for the log framework itself
 foreign+downcall: Generation of foreign linker downcall stubs
 foreign+upcall: Generation of foreign linker upcall stubs


Testing: manual testing + build on Windows with MSVC, and on Linux (WSL) with GCC.

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Commit messages:
 - add foreign linker tag set descriptions

Changes: https://git.openjdk.org/jdk/pull/13051/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13051&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304288
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13051.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13051/head:pull/13051

PR: https://git.openjdk.org/jdk/pull/13051