RFR: 8367862: debug.cpp: Do not print help message for methods not compiled in

[Kerem Kat]

The `help` command in `debug.cpp` was out of date, listing only a fraction of the available functions. This and other commands callable from gdb via `call help()`.

I added all commands to the help message, except `pns2` as it is documented as not being useful when called from gdb.

Also fixed is the message for the conditional `pns` command appearing in PRODUCT builds.

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Commit messages:
 - Add all debug.cpp commands to its help
 - Do not print pns help in PRODUCT builds

Changes: https://git.openjdk.org/jdk/pull/27341/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27341&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367862
  Stats: 36 lines in 1 file changed: 24 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/27341.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27341/head:pull/27341

PR: https://git.openjdk.org/jdk/pull/27341