gdb: ensure the cast in gdbarch_tdep is valid

This commit makes use of gdb::checked_static_cast when casting the
generic gdbarch_tdep pointer to a specific sub-class type.  This means
that, when compiled in developer mode, GDB will validate that the cast
is correct.

In order to use gdb::checked_static_cast the types involved must have
RTTI, which is why the gdbarch_tdep base class now has a virtual
destructor.

Assuming there are no bugs in GDB where we cast a gdbarch_tdep pointer
to the wrong type, then there should be no changes after this commit.

If any bugs do exist, then GDB will now assert (in a developer build).