Add option for test script to build for specified target

Use --target of -t to specify target we want to build.

Test: python test_compiler.py ../../../master
../../../master/prebuilts/clang/host/linux-x86/clang-stable --build-only
--target aosp_bullhead-eng
Bug: http://b/62292583

Change-Id: I28eb2d845154cb21f3eab29197572fe4ec9b34ce