MIPS: Reduce Baker read barrier code size overhead

Test: booted MIPS64 (with 2nd arch MIPS32R6) in QEMU
Test: test-art-target-gtest
Test: testrunner.py --target --optimizing
Test: same tests as above on CI20
Test: booted MIPS32 and MIPS64 in QEMU with poisoning
      in configurations:
      - with Baker read barrier thunks
      - without Baker read barrier thunks
      - ART_READ_BARRIER_TYPE=TABLELOOKUP

Change-Id: I79f320bf8862a04215c76cfeff3118ebc87f7ef2