ztest: make arc/cpu.h override C++ safe

This override header declares structures and inline functions, which
must be given C language linkage to avoid conflicting declaration
errors with other headers.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

subsys/testsuite/ztest/include/arch/cpu.h

@@ -6,6 +6,10 @@
 
 /* This file exists as a hack around Zephyr's dependencies */
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /* Architecture thread structure */
 struct _callee_saved {
 };
@@ -38,6 +42,10 @@ static inline bool arch_irq_unlocked(unsigned int key)
 	return 0;
 }
 
+#ifdef __cplusplus
+}
+#endif /* __cplusplus */
+
 #include <sys/arch_interface.h>