Revert "Define 64-bits atomics unconditionally"
Revert "Define 64-bits atomics unconditionally"

This reverts commit c2942e2c0e097e7c75a3addd0b9c87758f91692e.

This resolves #495.

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