Define HAVE_ATOMIC_BUILTINS and INNODB_RW_LOCKS_USE_ATOMICS in os0sync.h
instead of in univ.i. The code expects os_*() macros to be present if
HAVE_ATOMIC_BUILTINS and INNODB_RW_LOCKS_USE_ATOMICS are defined. So define
them next to defining the os_*() macros.