gdb: check for groups with duplicate names in reggroups:add

gdb: check for groups with duplicate names in reggroups:add

In the downstream ROCm GDB port, we would create multiple register
groups with duplicate names.  While it did not really hurt, it certainly
wasn't the intent.  And I don't think it ever makes sense to do so.

To catch these, change the assert in reggroups::add to check for
duplicate names.  It's no longer necessary to check for duplicate
reggroup pointers, because adding the same group twice would be caught
by the duplicate name check.

Change-Id: Id216a58acf91f1b314d3cba2d02de73656f8851d
Approved-By: Tom Tromey <tom@tromey.com>