unr64: use locked variant if not LP64

The current ifdefs are not sufficient to distinguish 32- and 64- bit
variants, which results e.g. in powerpc64 not using atomics.

While some 32-bit archs provide 64-bit atomics, there is no huge advantage
of using them on these platforms.

Reported by: many
Suggested by: jhb
Sponsored by: The FreeBSD Foundation