Add platform to get_so_extension on non-x86
The code in get_so_extension should add the canonical arch name on non-x86 architectures as well. For aarch64,
['.cpython-36m-aarch64-linux-gnu.so', '.abi3.so', '.so'] on cpython/aarch64 but
['.pypy37-pp73-linux-gnu.so'] on pypy. We cannot change this for current releases, but should fix it going forward.
Originally pointed out be @isuruf