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)

(cherry-picked from AOSP master
83c5612e)

Bug: 18035729

(cherry picked from commit f7ccfd00)

Change-Id: I5dca27241f46f481515b96e968fb2bef7866c89b