arch/common: rearrange for standard use of extern "C"

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

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 <[email protected]>

Author: pabigot

include/arch/common/ffs.h

@@ -8,13 +8,14 @@
 #ifndef ZEPHYR_INCLUDE_ARCH_COMMON_FFS_H_
 #define ZEPHYR_INCLUDE_ARCH_COMMON_FFS_H_
 
+#ifndef _ASMLANGUAGE
+
+#include <zephyr/types.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-#ifndef _ASMLANGUAGE
-
-#include <zephyr/types.h>
 /**
  *
  * @brief find most significant bit set in a 32-bit word
@@ -54,12 +55,10 @@ static ALWAYS_INLINE unsigned int find_lsb_set(u32_t op)
 	return __builtin_ffs(op);
 }
 
-#endif /* _ASMLANGUAGE */
-
-
 #ifdef __cplusplus
 }
 #endif
 
+#endif /* _ASMLANGUAGE */
 
 #endif /* ZEPHYR_INCLUDE_ARCH_COMMON_FFS_H_ */

include/arch/common/sys_io.h

@@ -12,15 +12,14 @@
 #ifndef ZEPHYR_INCLUDE_ARCH_COMMON_SYS_IO_H_
 #define ZEPHYR_INCLUDE_ARCH_COMMON_SYS_IO_H_
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #ifndef _ASMLANGUAGE
 
 #include <zephyr/types.h>
 #include <sys/sys_io.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 static ALWAYS_INLINE u8_t sys_read8(mem_addr_t addr)
 {
@@ -140,10 +139,10 @@ static ALWAYS_INLINE
 	return ret;
 }
 
-#endif
-
 #ifdef __cplusplus
 }
 #endif
 
+#endif /* _ASMLANGUAGE */
+
 #endif /* ZEPHYR_INCLUDE_ARCH_COMMON_SYS_IO_H_ */