New GDB symtab PR category

[Elena Zannoni]

FYI:

There is also a new symtab (symbol table + debug info) category in the
gnats database.

I did a quick sorting of PRs into this bucket (34).

There are 409 PRs left in the 'gdb' catch all category. If somebody
has a few spare cycles, it would be nice to start filling the various
buckets. Looking through the bug database should become less daunting,
after the sorting is done.

Elena