[Fuchsia] Use thread-pointer ABI slots for stack-protector and safe-stack

[Fuchsia] Use thread-pointer ABI slots for stack-protector and safe-stack

The Fuchsia ABI defines slots from the thread pointer where the
stack-guard value for stack-protector, and the unsafe stack pointer
for safe-stack, are stored. This parallels the Android ABI support.

Patch by Roland McGrath

Differential Revision: https://reviews.llvm.org/D30237

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@296081 91177308-0d34-0410-b5e6-96231b3b80d8