RFR: debug.cpp: Do not print help message for methods ifdef'd out [v2]

[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 The `help` command in `debug.cpp` was out of date, listing only a fraction of the available functions.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.

Kerem Kat has updated the pull request incrementally with one additional commit since the last revision:

  address comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/27341/files
  - new: https://git.openjdk.org/jdk/pull/27341/files/e1bdce21..6077d252

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27341&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27341&range=00-01

  Stats: 27 lines in 1 file changed: 1 ins; 0 del; 26 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