MIPS: Adjust LibartImgDeviceBaseAddress() for CC GC.

The concurrent copying GC allocates large contiguous
space at low addresses. This moves the image(s) out
of the way to higher addresses.

Test: booted MIPS32R2 in QEMU
Test: booted MIPS64 (with 2nd arch MIPS32R2) in QEMU

Change-Id: I1c54e6142844ab1ad7820765ae313650e9ab8875