[C++] Use mimalloc as the default memory allocator on macOS

This is a follow on to ARROW-12316 which was supposed to do that (and I thought it did!) but now I'm suspecting we actually wanted that if logic at https://github.com/apache/arrow/blob/master/cpp/src/arrow/memory_pool.cc#L168-L180 instead/as well or even better use the first element in SupportedBackends() vector.