Fix ReadBarrier misuse when relocating the image.

Specify not to use ReadBarrier when calling to GetSuperClass during
relocation.

Bug: 63691502
Test: test/testrunner/testrunner.py -b --host -t \
      test-art-host-run-test-debug-prebuild-optimizing-no-relocate-\
      ntrace-gcstress-checkjni-picimage-npictest-ndebuggable-no-jvmti-\
      626-const-class-linking64

Change-Id: I9fbfdd4ffa2c7489da2e3c62af4f196ec07e562e