gdb: C++-ify internal_function

Change the `name` field to std::string, add constructor.  Remove
function `create_internal_function`, since it becomes a trivial wrapper
around the constructor.

Change-Id: Ifc8b1282c442e1930bcd69d6e140128067e49563
Reviewed-By: Guinevere Larsen <guinevere@redhat.com>