include: rearrange for standard use of extern "C" in various headers

Consistently place C++ use of extern "C" after all include directives,
within the negative branch of _ASMLANGUAGE if used.

The inclusion of the generated syscall files is placed outside the
extern "C" block as the generated file has its own extern "C" block.

Background from issue #17997:

Declarations that use C linkage should be placed within extern "C"
so the language linkage is correct when the header is included by
a C++ compiler.

Similarly #include directives should be outside the extern "C" to
ensure the language-specific default linkage is applied to any
declarations provided by the included header.

See: https://en.cppreference.com/w/cpp/language/language_linkage
Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

include/sys/atomic.h

@@ -456,9 +456,6 @@ static inline void atomic_set_bit_to(atomic_t *target, int bit, bool val)
 	}
 }
 
-#ifdef CONFIG_ATOMIC_OPERATIONS_C
-#include <syscalls/atomic.h>
-#endif
 /**
  * @}
  */
@@ -467,4 +464,8 @@ static inline void atomic_set_bit_to(atomic_t *target, int bit, bool val)
 }
 #endif
 
+#ifdef CONFIG_ATOMIC_OPERATIONS_C
+#include <syscalls/atomic.h>
+#endif
+
 #endif /* ZEPHYR_INCLUDE_SYS_ATOMIC_H_ */

include/sys/sys_io.h

@@ -9,13 +9,13 @@
 #ifndef ZEPHYR_INCLUDE_SYS_SYS_IO_H_
 #define ZEPHYR_INCLUDE_SYS_SYS_IO_H_
 
+#include <zephyr/types.h>
+#include <stddef.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-#include <zephyr/types.h>
-#include <stddef.h>
-
 typedef u32_t io_port_t;
 typedef u32_t mm_reg_t;
 typedef uintptr_t mem_addr_t;