panic: sys_info: rewrite a fix for a compilation error (`make W=1`)

panic: sys_info: rewrite a fix for a compilation error (`make W=1`)

Compiler was not happy about dead variable in use:

lib/sys_info.c:52:19: error: variable 'sys_info_avail' is not needed and will not be emitted [-Werror,-Wunneeded-internal-declaration]
   52 | static const char sys_info_avail[] = "tasks,mem,timers,locks,ftrace,all_bt,blocked_tasks";
      |                   ^~~~~~~~~~~~~~

This was fixed by adding __maybe_unused attribute that just hides the
issue and didn't actually fix the root cause.  Rewrite the fix by moving
the local variable from stack to a heap.

As a side effect this drops unneeded "synchronisation" of duplicative info
and also makes code ready for the further refactoring.

Link: https://lkml.kernel.org/r/20251030132007.3742368-5-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Cc: Feng Tang <feng.tang@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>