Disable clang optimization to boot up on arm64.

On arm64 devices, clang compiled lib64/libart.so repeatedly
crashes during boot up stage. That prevents a stable adb connection.
When optimization of BitVector::NumSetBits is disabled,
we can boot up to Android home screen, although some Apps
are still unstable.
This is a temporary workaround only for clang/llvm arm64 target,
to enable concurrent debugging on other bugs.

BUG: 19180814
Change-Id: Id82497ce4be0b2c30d36679d15394848d826f50c