[Driver] Fuchsia targets default to -fasynchronous-unwind-tables

This regressed for x86-64 in r307856 because it's no longer inherited
from Generic_GCC. We'd never noticed that it was missing other
targets (i.e. aarch64), but Fuchsia is uniform across all machines.

Patch by Roland McGrath

Differential Revision: https://reviews.llvm.org/D37723

llvm-svn: 312989