Use builtin type when appropriate

Use builtin type when appropriate

There are a few spots that check whether a type is objfile-owned, and
then choose either the objfile- or arch-specific builtin type.  I
don't think there is a need to do this any more (if there ever was),
because it is ok for an objfile-allocated type to refer to an
arch-allocated type.

Reviewed-By: Simon Marchi <simon.marchi@efficios.com>