[asan] Fix SELinux setup on Android-K.

On pre-L devices SELinux is set up in such a way that zygote wrapper
has to be in system_file context, not zygote_exec.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@218599 91177308-0d34-0410-b5e6-96231b3b80d8