Prebuilt clang: add a TARGET_ENABLE_PREBUILT_CLANG flag, which sets TARGET_CLANG_VERSION to msm-3.4

simply setting TARGET_CLANG_VERSION still enables use of prebuilt clang, as before this commit.

Change-Id: Ib8cb6312d9a7f9ffff3e48c5c66bfb80893bcf97