gdb: add type::target_type / type::set_target_type

gdb: add type::target_type / type::set_target_type

Add the `target_type` and `set_target_type` methods on `struct type`, in order
to remove the `TYPE_TARGET_TYPE` macro.  In this patch, the macro is changed to
use the getter, so all the call sites of the macro that are used as a setter
are changed to use the setter method directly.  The next patch will remove the
macro completely.

Change-Id: I85ce24d847763badd34fdee3e14b8c8c14cb3161