MIPS: Shorten .bss string/class loads

This is a follow-up to
https://android-review.googlesource.com/#/c/384033/.

Test: booted MIPS64 (with 2nd arch MIPS32R6) in QEMU
Test: testrunner.py --target --optimizing
Test: same tests as above on CI20
Test: booted MIPS32R2 and MIPS64 in QEMU in configurations:
      ART_USE_READ_BARRIER=false,
      ART_READ_BARRIER_TYPE=TABLELOOKUP

Change-Id: I4cb2f4ded13c0d9fc960c7eac55396f7931c1e38