Split the atomic implementation into three files:
- arch_atomic.h: Shared macros (STORE, LOAD, etc.) using atomic_lock()/
atomic_unlock() abstraction
- arch_atomic_irq.c: 32-bit atomic functions using IRQ disable (conditional
on CONFIG_LIBC_ATOMIC_IRQ via Make.defs)
- arch_atomic64.c: 64-bit atomic functions using spinlock (always compiled,
multi-core safe). The __atomic_* functions are always provided (GCC
runtime helpers), while nx_atomic_* functions are conditional on
!CONFIG_LIBC_ATOMIC_TOOLCHAIN.
Signed-off-by: zhangyu117 <zhangyu117@xiaomi.com>
