debug: implement symtab generation

Use pyelftools to extract the symbol table from the
link stage executable. Then, filter out the function names
and sort them based on their offsets before writing into the
`symtab.c`, this is similar to how the `isr_tables` works.

To access the structure, simply include the new header:
```c
#include <zephyr/debug/symtab.h>
```

Signed-off-by: Yong Cong Sin <ycsin@meta.com>