gdb: make iterate_over_symbols return void, rename to for_each_symbol

gdb: make iterate_over_symbols return void, rename to for_each_symbol

Nothing really uses the return value of iterate_over_symbols and
language::iterate_over_symbols.  Also, all provided callback always
return true, iterating on all matching symbols.  Simplify them to not
return a value and not have the "stop iterating" feature.

Rename to for_each_symbol, just to be consistent with previous patches.

Also rename symbol_found_callback_ftype to
for_each_symbol_callback_ftype for consistency.

Change-Id: I55ff3162098bb069dc1de1afca10dd9abfc05c34
Approved-By: Andrew Burgess <aburgess@redhat.com>