MIPS32: Improve string and class loads

Tested:
- MIPS32 Android boots in QEMU
- test-art-host-gtest
- test-art-target-run-test-optimizing in QEMU, on CI20
- test-art-target-gtest on CI20

Change-Id: I70fd5d5267f8594c3b29d5a4ccf66b8ca8b09df3
14 files changed