[Driver] Enable ShadowCallStack, not SafeStack, by default on AArch64 Fuchsia

Submitted for mcgrathr.

On AArch64, Fuchsia fully supports both SafeStack and ShadowCallStack ABIs.
The latter is now preferred and will be the default. It's possible to
enable both simultaneously, but ShadowCallStack is believed to have most
of the practical benefit of SafeStack with less cost.

Differential Revision: https://reviews.llvm.org/D66712