Use binder bitness instead of primary arch for references root.

Binder bitness does not always directly relate to primary arch as
thought before. For eg: a target with primary arch 'arm' could be using
64 bit binder.

Bug: 74362434

Test: utils/create_reference_dumps.py

Change-Id: I1c1214b538e67247c666307d1360e4260239e4e1