gdb: convert reggroups to use a std::vector

Replace manual linked list with a std::vector.  This commit doesn't
change the reggroup_next and reggroup_prev API, but that will change
in a later commit.

This commit is focused on the minimal changes needed to manage the
reggroups using a std::vector, without changing the API exposed by the
reggroup.c file.

There should be no user visible changes after this commit.