[asan] asan_device_setup improvements.

* Allow user SEGV handler if Android-L-like setup is detected.
  Necessary for correctness.
* Change file context labels on the wrapper script to appease SELinux.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@218124 91177308-0d34-0410-b5e6-96231b3b80d8