Android support for SafeStack.
Add two new ways of accessing the unsafe stack pointer:
- At a fixed offset from the thread TLS base. This is very similar to StackProtector cookies, but we plan to extend it to other backends (ARM in particular) soon. Bionic-side implementation here: https://android-review.googlesource.com/170988.
- Via a function call, as a fallback for platforms that provide neither a fixed TLS slot, nor a reasonable TLS implementation (i.e. not emutls).
This is a re-commit of a change in r248357 that was reverted in