MIPS: Implement read barriers.

This is the core functionality. Further improvements
will be done separately.

This also adds/moves memory barriers where they belong and
removes the UnsafeGetLongVolatile and UnsafePutLongVolatile
MIPS32 intrinsics as they need to load/store a pair of
registers atomically, which is not supported directly by
the CPU.

Test: booted MIPS32R2 in QEMU
Test: test-art-target-run-test
Test: booted MIPS64 (with 2nd arch MIPS32R6) in QEMU
Test: "testrunner.py --target --optimizing -j1"
Test: same MIPS64 boot/test with ART_READ_BARRIER_TYPE=TABLELOOKUP
Test: "testrunner.py --target --optimizing --32 -j2" on CI20
Test: same CI20 test with ART_READ_BARRIER_TYPE=TABLELOOKUP

Change-Id: I0ff91525fefba3ec1cc019f50316478a888acced