kernel: prioritize C atomic functions if selected

Usually, GCC builtin or arch-specific atomic functions are being
used. The corresponding kconfigs are "selected" by architecture
or SoC kconfigs. CONFIG_ATOMIC_OPERATIONS_C is usually used to
override the GCC built-in or arch-specific atomic functions when
these two are not supported. So change the priority of #include
so that C version is included first if selected, and skips
the inline versions of the other two variants. Or else there
will be two compiled versions of atomic functions: inline version
and the compiled C version. Note that the arch-specific version
and builtin are swapped, so the builtin one is now the default.

Fixes zephyrproject-rtos#33857

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

include/sys/atomic.h

@@ -24,18 +24,18 @@ typedef void *atomic_ptr_t;
 
 /* Low-level primitives come in several styles: */
 
-#if defined(CONFIG_ATOMIC_OPERATIONS_BUILTIN)
-/* Default.  See this file for the Doxygen reference: */
-#include <sys/atomic_builtin.h>
+#if defined(CONFIG_ATOMIC_OPERATIONS_C)
+/* Generic-but-slow implementation based on kernel locking and syscalls */
+#include <sys/atomic_c.h>
 #elif defined(CONFIG_ATOMIC_OPERATIONS_ARCH)
 /* Some architectures need their own implementation */
 # ifdef CONFIG_XTENSA
 /* Not all Xtensa toolchains support GCC-style atomic intrinsics */
 # include <arch/xtensa/atomic_xtensa.h>
 # endif
 #else
-/* Generic-but-slow implementation based on kernel locking and syscalls */
-#include <sys/atomic_c.h>
+/* Default.  See this file for the Doxygen reference: */
+#include <sys/atomic_builtin.h>
 #endif
 
 /* Portable higher-level utilities: */