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/fs/fcb.h

@@ -7,10 +7,6 @@
 #ifndef ZEPHYR_INCLUDE_FS_FCB_H_
 #define ZEPHYR_INCLUDE_FS_FCB_H_
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 /*
  * Flash circular buffer.
  */
@@ -21,6 +17,10 @@ extern "C" {
 
 #include <kernel.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  * @defgroup fcb Flash Circular Buffer (FCB)
  * @ingroup file_system_storage