envsetup: block SDCLANG on Darwin for now

Snapdragon LLVM is not supported on Darwin yet.
There might be the case where the directory structure is set up
by default and the build system will try to execute the linux binaries.

This will fail obviously:
  /bin/bash: /Volumes/Android/build/prebuilts/snapdragon-llvm/toolchains/llvm-Snapdragon_LLVM_for_Android_3.8/prebuilt/linux-x86_64/bin/clang: cannot execute binary file

Change-Id: I1f4d0bbda88b540b7ac3159f8e83de4501b9073b
Signed-off-by: Alexander Martinz <eviscerationls@gmail.com>