libtorrent: use the proper BR2_TOOLCHAIN_HAS_SYNC_x symbols

libtorrent uses 1 bytes and 4 bytes __sync built-ins, so let's use the
appropriate BR2_TOOLCHAIN_HAS_SYNC_x symbols instead of
BR2_ARCH_HAS_ATOMICS. The rtorrent package, which is a reverse
dependency of libtorrent, gets fixed as well.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr: build-tested on an ARC toolchain without
 atomics, with CPU 770D]
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>