MIPS: Improve object lock/unlock.

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: repeat all of the above in the configuration
      ART_USE_READ_BARRIER=false
Test: booted MIPS32R2 in QEMU

Change-Id: I30fa277e1a687cb0595204e8d9053d7a4873d3df