include: sys: atomic: Check for ATOMIC_OPERATIONS_BUILTIN

The code today relied on ATOMIC_OPERATIONS_BUILTIN being the default
option if the other alternatives (_C and _ARCH) were not enabled.
Instead, explicitly check for it to avoid confusion when browsing the
code and to ensure that the user can always match a particular Kconfig
option with the #ifdeffery in the header file.

Signed-off-by: Carles Cufi <[email protected]>

Author: carlescufi

include/zephyr/sys/atomic.h

@@ -35,9 +35,11 @@ extern "C" {
 /* Other arch specific implementation */
 # include <zephyr/sys/atomic_arch.h>
 # endif /* CONFIG_XTENSA */
-#else
+#elif defined(CONFIG_ATOMIC_OPERATIONS_BUILTIN)
 /* Default.  See this file for the Doxygen reference: */
 #include <zephyr/sys/atomic_builtin.h>
+#else
+#error "CONFIG_ATOMIC_OPERATIONS_* not defined"
 #endif
 
 /* Portable higher-level utilities: */