RTEMS | Libdebugger register block corruption (#5634)

[Chris Johns (@chris)]

Issue created by Chris Johns: https://gitlab.rtems.org/rtems/rtos/rtems/-/work_items/5634

Assignee: Chris Johns

## Summary

If more than 32 threads are active the `thread->register` pointer may end up referencing an old block pointer depending on the value returned by a `realloc` call. This has not been a problem on ARM or AARCH64 builds but is a problem on PowerPC although it is a definite issue on all architectures. 

I suspect a key difference is PowerPC has a 412 byte register buffer per thread, significantly more than ARM and AARCH64, so a block resize using `realloc` is more likely to results in the pointer returned having a different address. The corruption is each thread has taken a pointer to the registers from the original block for each thread and it is not re-based after the `realloc` moves the pointer.

## Solution

Add a rebase handler to the block code that is called if the base pointer of a block moves so the pointer can be remapped.

_Note_, only the `registers` is effected as all other blocks do not have references taken.

## Steps to reproduce

Run a simple EPICS IOC application on PowerPC, connect then list the threads and then print thread's registers. The values are garbage.

-- 
View it on GitLab: https://gitlab.rtems.org/rtems/rtos/rtems/-/work_items/5634
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