Disable ptype/o for dynamic types

A user pointed out that "ptype/o" of a certain Ada type -- while in C
mode -- caused gdb to crash.

The bug here is that dynamic types can't really be printed this way.
This patch avoids the bug by disabling the "/o" feature in this case.

Note that using "ptype/o" in this way makes sense for the time being,
because the Ada code doesn't support the "/o" feature (yet); and in
any case gdb should not crash.