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/sw_isr_table.h

@@ -14,14 +14,14 @@
 #ifndef ZEPHYR_INCLUDE_SW_ISR_TABLE_H_
 #define ZEPHYR_INCLUDE_SW_ISR_TABLE_H_
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #if !defined(_ASMLANGUAGE)
 #include <zephyr/types.h>
 #include <toolchain.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /*
  * Note the order: arg first, then ISR. This allows a table entry to be
  * loaded arg -> r0, isr -> r3 in _isr_wrapper with one ldmia instruction,
@@ -75,10 +75,10 @@ struct _isr_list {
 void z_isr_install(unsigned int irq, void (*routine)(void *), void *param);
 #endif
 
-#endif /* _ASMLANGUAGE */
-
 #ifdef __cplusplus
 }
 #endif
 
+#endif /* _ASMLANGUAGE */
+
 #endif /* ZEPHYR_INCLUDE_SW_ISR_TABLE_H_ */