arch/arm: 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.

In arch.h the extern "C" in the including context is left active during
include of target-specific mpu headers to avoid more complex
restructuring.

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/arm/asm_inline_gcc.h

@@ -11,10 +11,6 @@
 #ifndef ZEPHYR_INCLUDE_ARCH_ARM_CORTEX_M_ASM_INLINE_GCC_H_
 #define ZEPHYR_INCLUDE_ARCH_ARM_CORTEX_M_ASM_INLINE_GCC_H_
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 /*
  * The file must not be included directly
  * Include arch/cpu.h instead
@@ -26,6 +22,10 @@ extern "C" {
 #include <arch/arm/exc.h>
 #include <irq.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /**
  *
  * @brief Disable all interrupts on the CPU
@@ -158,10 +158,10 @@ static ALWAYS_INLINE bool z_arch_irq_unlocked(unsigned int key)
 	return key == 0;
 }
 
-#endif /* _ASMLANGUAGE */
-
 #ifdef __cplusplus
 }
 #endif
 
+#endif /* _ASMLANGUAGE */
+
 #endif /* ZEPHYR_INCLUDE_ARCH_ARM_CORTEX_M_ASM_INLINE_GCC_H_ */