It's either "GNU *Library* General Public version 2" or "GNU Lesser
General Public version *2.1*", but there was no "version 2.0" of the
"Lesser" library. So assume that version 2.1 is meant here.
Backports commit fb0343d5b4dd4b9b9e96e563d913a3e0c709fe4e from qemu
GCC7+ will no longer advertise support for 16-byte __atomic operations
if only cmpxchg is supported, as for x86_64. Fortunately, x86_64 still
has support for __sync_compare_and_swap_16 and we can make use of that.
AArch64 does not have, nor ever has had such support, so open-code it.
Backports commit e6cd4bb59b8154fa00da611200beef7eb4e8ec56 from qemu
Given that this atomic operation will be used by both risc-v
and aarch64, let's not duplicate code across the two targets.
Backports commit 5507c2bf35aa6b4705939349184e71afd5e058b2 from qemu
