Cleanup Android build and provide better diagnostics

No longer need to set ANDROID if COMPILER_RT_TEST_TARGET_TRIPLE is
arm-linux-androideabi.

No need to set ANDROID_COMMON_FLAGS.  These flags are already in
CMAKE_CXX_FLAGS which are used in try_compile().

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@210053 91177308-0d34-0410-b5e6-96231b3b80d8