Reintroduce JEMALLOC_ATOMIC_U64
Reintroduce JEMALLOC_ATOMIC_U64

The C11 atomics backport removed this #define, which degraded atomic 64-bit
reads to require a lock even on platforms that support them.  This commit fixes
that.

wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc,wqfish/jemalloc