[test] pass TARGET_THUMB_STRICT and -O3 to clang

clang is used to compile mostly graphics related code in 4.4.X.
Upstream is moving to replace GCC with a newer clang/llvm, so we
should start getting familiar with it.. and perhaps switch to a
newer drop-in prebuilt with polly support soon

-VanirAOSP

Change-Id: Ic8eb6be6ba3a9cd472a4ec9c34c515c1f25d167e