String Compression for MIPS32 and MIPS64

Changes on intrinsics and Code Generation on MIPS32 and MIPS64 for
string compression feature.

Testing is done with STRING_COMPRESSION_ENABLED = true (in libcore),
mirror::kUseStringCompression = true and STRING_COMPRESSION_FEATURE set
to 1.

Test: booted MIPS32 and MIPS64 in QEMU
Test: mma test-art-target-run-test on CI20 (MIPS32R2)
Test: mma test-art-target-run-test in QEMU (MIPS64R6)

Change-Id: If50a6b6c0792bfa34d4fdff6bf2c7542211d2689