arch/nios2: 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.

Remove extern "C" support from files that don't declare objects or
functions.

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/arch/nios2/arch.h

@@ -22,10 +22,6 @@
 #include <arch/common/sys_io.h>
 #include <arch/common/ffs.h>
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #define STACK_ALIGN  4
 
 #ifndef _ASMLANGUAGE
@@ -33,6 +29,9 @@ extern "C" {
 #include <irq.h>
 #include <sw_isr_table.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 /**
  * Configure a static interrupt.
@@ -210,10 +209,10 @@ static ALWAYS_INLINE void arch_nop(void)
 	__asm__ volatile("nop");
 }
 
-#endif /* _ASMLANGUAGE */
-
 #ifdef __cplusplus
 }
 #endif
 
-#endif
+#endif /* _ASMLANGUAGE */
+
+#endif /* ZEPHYR_INCLUDE_ARCH_NIOS2_ARCH_H_ */