top: get rid of on-stack variable buffers, use permanent one.

top: get rid of on-stack variable buffers, use permanent one.
code shrank with and without TOPMEM:

top_main                                             828     844     +16
display_process_list                                1525    1473     -52
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/1 up/down: 16/-52)            Total: -36 bytes

top_main                                            1150    1171     +21
display_topmem_process_list                         1150    1167     +17
display_process_list                                1525    1473     -52
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/1 up/down: 38/-52)            Total: -14 bytes