libbpf: Fix arm64 build

The macro for libbpf_smp_store_release() doesn't build on arm64, fix it.

Fixes: 291471dd ("libbpf, xsk: Add libbpf_smp_store_release libbpf_smp_load_acquire")
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210308182521.155536-1-jean-philippe@linaro.org