arch/x86: 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 <peter.bigot@nordicsemi.no>

arch/x86/include/ia32/kernel_arch_func.h

@@ -12,6 +12,8 @@
 
 #ifndef _ASMLANGUAGE
 
+#include <stddef.h> /* For size_t */
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -92,8 +94,6 @@ static inline struct x86_mmu_pdpt *z_x86_pdpt_get(struct k_thread *thread)
 /* ASM code to fiddle with registers to enable the MMU with PAE paging */
 void z_x86_enable_paging(void);
 
-#include <stddef.h> /* For size_t */
-
 #ifdef __cplusplus
 }
 #endif

include/arch/x86/ia32/arch.h

@@ -25,13 +25,12 @@
 #include <misc/util.h>
 
 #ifndef _ASMLANGUAGE
+#include <stddef.h>	/* for size_t */
+
 #include <arch/common/addr_types.h>
 #include <arch/x86/ia32/segmentation.h>
-#endif
 
-#ifdef __cplusplus
-extern "C" {
-#endif
+#endif /* _ASMLANGUAGE */
 
 /* GDT layout */
 #define CODE_SEG	0x08
@@ -53,9 +52,13 @@ extern "C" {
 
 #ifndef _ASMLANGUAGE
 
-/* interrupt/exception/error related definitions */
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 
+/* interrupt/exception/error related definitions */
+
 /*
  * The TCS must be aligned to the same boundary as that used by the floating
  * point register set.  This applies even for threads that don't initially
@@ -315,10 +318,6 @@ struct _x86_syscall_stack_frame {
 	u32_t ss;
 };
 
-#endif /* !_ASMLANGUAGE */
-
-#ifndef _ASMLANGUAGE
-
 /**
  *
  * @internal
@@ -561,8 +560,6 @@ extern void k_float_enable(struct k_thread *thread, unsigned int options);
  * @}
  */
 
-#include <stddef.h>	/* for size_t */
-
 extern void	k_cpu_idle(void);
 
 extern u32_t z_timer_cycle_get_32(void);
@@ -809,10 +806,10 @@ int z_x86_mmu_validate(struct x86_mmu_pdpt *pdpt, void *addr, size_t size,
 		       int write);
 #endif /* CONFIG_X86_MMU */
 
-#endif /* !_ASMLANGUAGE */
-
 #ifdef __cplusplus
 }
 #endif
 
+#endif /* !_ASMLANGUAGE */
+
 #endif /* ZEPHYR_INCLUDE_ARCH_X86_IA32_ARCH_H_ */