arch: arm: Fix cxd56xx for SMP

Summary:
- In apache#14465,
  atomic_compare_exchange_weak_explicit() was newly introduced
  in semaphore. However, cxd56xx has an issue with the API
  if SMP is enabled (see up_testset2 in cxd56_testset.c).
- This commit fixes the issue by using LIBC_ARCH_ATOMIC.

Impact:
- Only cxd56xx SoCs in SMP mode.

Testing:
- Tested with spresense:smp, spresense:wifi_smp
- NOTE: If DEBUG_ASSERTIONS is enabled assert would be happend.
  I think this might be another issue.

Signed-off-by: Masayuki Ishikawa <[email protected]>

arch/arm/Kconfig

@@ -672,6 +672,7 @@ config ARCH_CHIP_CXD56XX
 	select ARCH_HAVE_MATH_H
 	select ARCH_HAVE_I2CRESET
 	select ARCH_HAVE_CUSTOM_VECTORS
+	select LIBC_ARCH_ATOMIC if SMP
 	---help---
 		Sony CXD56XX (ARM Cortex-M4) architectures