Summary:
Use clang-5.0 -Oz as default NDK compiler for android.
It does 3 good things:
* brings us into compliance with deprecation of gcc for android NDKs since r13
* yields slightly smaller native code, on the order of many-10s-to-small-100s of KB
* slight improvement in startup performance, on the order of 10s of ms
Reviewed By: mzlee
Differential Revision: D6155682
fbshipit-source-id: 2f64e742f4dc44be171274c2ad0a41fb1e7079d7
0c98e89fdb