[CMake] Avoid internal dependencies in the runtimes build
[CMake] Avoid internal dependencies in the runtimes build

When performing runtimes build, dependencies like clang and
clang-headers aren't available. This was accidentally omitted in D57992.

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@353796 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt