Work better with how make/bmake works:

Work better with how make/bmake works:
1.  Avoid a cd back into ${.CURDIR} to run mkbuiltins when we know make
    will first cd into ${.OBJDIR}. Keep the cwd to what make sets it to.
2.  Don't tell mkbuiltins where to write to (= ${.OBJDIR}), but where to
    get sources from (= ${.CURDIR}). This to compensate for point 1.

This fixes a problem with bmake's mk files that optimize ${.OBJDIR} to
expand to "." after changing cwd, not taking into account that the
target is pretty much undoing that and not getting the full path to the
object tree anymore.