gdb: add some additional thread status debug output

While working on this patch:

  https://sourceware.org/pipermail/gdb-patches/2022-January/185109.html

I found it really useful to print the executing/resumed status of all
threads (or all threads in a particular inferior) at various
places (e.g. when a new inferior is started, when GDB attaches, etc).

This debug was originally part of the above patch, but I wanted to
rewrite this as a separate patch and move the code into a new function
in infrun.h, which is what this patch does.

Unless 'set debug infrun on' is in effect, then there should be no
user visible changes after this commit.

gdb/infrun.c

@@ -5063,6 +5063,9 @@ stop_all_threads (const char *reason, inferior *inf)
   INFRUN_SCOPED_DEBUG_START_END ("reason=%s, inf=%d", reason,
 				 inf != nullptr ? inf->num : -1);
 
+  infrun_debug_show_threads ("non-exited threads",
+			     all_non_exited_threads ());
+
   scoped_restore_current_thread restore_thread;
 
   /* Enable thread events on relevant targets.  */