ARM: VIXL32: Implement read barriers.

Test: ART_USE_VIXL_ARM_BACKEND=true {ART_USE_READ_BARRIER=true, \
      ART_USE_READ_BARRIER=true ART_HEAP_POISONING=true, \
      ART_USE_READ_BARRIER=true ART_READ_BARRIER_TYPE=TABLELOOKUP, \
      ART_USE_READ_BARRIER=true ART_READ_BARRIER_TYPE=TABLELOOKUP \
      ART_HEAP_POISONING=true, ART_HEAP_POISONING=true} \
      m test-art-host
Test: ART_USE_VIXL_ARM_BACKEND=true {ART_USE_READ_BARRIER=true, \
      ART_USE_READ_BARRIER=true ART_HEAP_POISONING=true, \
      ART_USE_READ_BARRIER=true ART_READ_BARRIER_TYPE=TABLELOOKUP, \
      ART_USE_READ_BARRIER=true ART_READ_BARRIER_TYPE=TABLELOOKUP \
      ART_HEAP_POISONING=true, ART_HEAP_POISONING=true} \
      m test-art-target
Change-Id: I2c1e4875beb09329c3b69bd5e1e8bbd3861d5866