MIPS64: Sign-extend results in art_quick_get32_static/instance

Return types of artGet32StaticFromCode and artGet32InstanceFromCode
are unsigned (size_t). On the other hand, managed code expects signed
32-bit values. This is problematic on MIPS64 which expects that all
integer type variables are properly sign-extended in registers.

This fixes test 529-checker-unresolved.

Test: mma -j2 test-art-target in QEMU
Change-Id: I70f53ded9bfa1cfd53bcc639fb4841192ee71b2b