GCC made the switch on 2018-04-10 ("rs6000: Enable -fasynchronous-unwind-tables by default").
In Clang, FreeBSD/NetBSD powerpc have already defaulted to -fasynchronous-unwind-tables.
This patch defaults Generic_GCC powerpc (which affects Linux) to use -fasynchronous-unwind-tables.
Reviewed By: #powerpc, nemanjai
Differential Revision: https://reviews.llvm.org/D92054