[Ada] Rename struct tcb_fieldnos into atcb_fieldnos.

[Ada] Rename struct tcb_fieldnos into atcb_fieldnos.

`atcb' is clearer than `tcb', as it stands for Ada Task Control
Block. This is also consistent with the rest of the code, which
uses atcb...

gdb/ChangeLog:

        * ada-tasks.c (struct atcb_fieldnos): Renames struct tcb_fieldnos.