MIPS: Refactor code for unresolved field entrypoint.

This is a follow-up to
https://android-review.googlesource.com/#/c/325423/.

Test: booted MIPS32R2 in QEMU
Test: test-art-target-run-test-optimizing in QEMU
Test: booted MIPS64 (with 2nd arch MIPS32R6) in QEMU
Test: test-art-target-run-test-optimizing
      (MIPS64R6 and MIPS32R6) in QEMU

Change-Id: Ie663ecf0489e7b182434708bef71686df5f37273