gdb: convert 'set debug symbol-lookup' to new debug printing scheme

gdb: convert 'set debug symbol-lookup' to new debug printing scheme

Convert the implementation of 'set debug symbol-lookup' to the new
debug printing scheme.

In a few places I've updated the debug output to remove places where
the printed debug message included the function name, the new debug
scheme already adds that, but I haven't done all the possible updates.