[Driver] Default Generic_GCC aarch64 to -fasynchronous-unwind-tables

In GCC, `aarch64-*-linux` and `aarch64-*-freebsd` made the switch in 2018
(https://gcc.gnu.org/pipermail/gcc-patches/2018-March/495549.html).
In Clang, FreeBSD/Fuchsia/NetBSD/MinGW aarch64 default to -fasynchronous-unwind-tables.

This patch defaults Generic_GCC aarch64 (which affects Linux) to use -fasynchronous-unwind-tables.

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D91760