zygote/dex2oat: Grant additional symlink permissions

* zygote needs to be able to symlink from dalvik cache to system
  to avoid having to copy boot.oat
  (when the boot.oat file was built with --compile-pic)
* dex2oat needs to be able to read the symlink in the dalvik cache
  (the one that zygote creates)

Bug: 18035729
Change-Id: Ie1acad81a0fd8b2f24e1f3f07a06e6fdb548be62