package/libatomic_ops: enable for riscv32 and riscv64

The riscv support has been added since v7.6.4 release.

https://github.com/ivmai/libatomic_ops/commit/137643f1417ded071c56e05401ca6a3c16492d96



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
[yann.morin.1998@free.fr:
  - split the long line
  - reorder the archs alphabetically, and group related ones
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>