Fix 64-bit alignment of 64-bit words accessed atomically on ARM

[buraksezer]

See caddyserver/cache-handler-11 for details.

From sync/atomic documentation:

On ARM, 386, and 32-bit MIPS, it is the caller's responsibility to arrange for 64-bit alignment of 64-bit words accessed atomically. The first word in a variable or in an allocated struct, array, or slice can be relied upon to be 64-bit aligned.