stack protector: use AT_RANDOM

Populate the stack canaries from the kernel supplied
AT_RANDOM value, which doesn't involve any system calls.
This is slightly faster (6 fewer syscalls) and avoids
unnecessarily reading /dev/urandom, which depletes entropy.

Bug: 7959813

Change-Id: If2b43100a2a9929666df3de56b6139fed969e0f1
6 files changed