MIPS: Check for forwarding address in READ_BARRIER_MARK_REG.

Also check for null and marked bit.

Bug: 30162165

Test: booted MIPS64 (with 2nd arch MIPS32R6) in QEMU
Test: test-art-target-gtest
Test: testrunner.py --target --optimizing -j1
Test: same tests as above on CI20
Test: booted MIPS32R2 in QEMU

Change-Id: I9c2987d0bce757b0e7e379a017b57a9107dd7c08
2 files changed