gdb/gdbserver: fix some defined but unused function warnings

This commit:

  commit 198ff6ff
  Date:   Tue Jan 30 15:37:23 2024 +0000

      gdb/gdbserver: share x86/linux tdesc caching

added some functions which are always defined, but their use is
guarded within various #ifdef blocks.  As a result we were seeing
errors about defined, but unused, functions.

I've fixed this problem in this commit by wrapping the function
definitions within #ifdef blocks.

I'm a little worried that there might be too many #ifdef blocks within
this file, however, I'm going to commit this fix for now as this will
fix the build, then I'll think about if there's a better way to split
this file so we might avoid some of these #ifdef blocks.