Use new atomic types from Go 1.19

This is a cleaner solution for the fix in #438 thanks to the fact that Go 1.19 now is the default and the atomic.Int64 types are automatically aligned correctly on 32 bit systems.

Using this also means that xsync.Int64 can be removed. The new atomic.Int64 type solves the issue and should be quite a lot faster as it avoids the interface conversion.