Move .art.rel data to a section in .art, part 1.

Refactor the code so that we can collect all relocation
offsets while writing the image.

Test: Manually verify that the aosp_taimen-userdebug boot
      image is identical before and after this change;
      using WIP follow-up code, check that we record the
      same locations as the patchoat-generated .rel files.
Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 77856493
Change-Id: Id8c78e417e77e859a82a680504253f41e84560d6