parisc: Fix compiler warnings in debug_core.c

Fix this compiler warning:
kernel/debug/debug_core.c: In function ‘kgdb_cpu_enter’:
arch/parisc/include/asm/cmpxchg.h:48:3: warning: value computed is not used [-Wunused-value]
   48 |  ((__typeof__(*(ptr)))__xchg((unsigned long)(x), (ptr), sizeof(*(ptr))))
arch/parisc/include/asm/atomic.h:78:30: note: in expansion of macro ‘xchg’
   78 | #define atomic_xchg(v, new) (xchg(&((v)->counter), new))
      |                              ^~~~
kernel/debug/debug_core.c:596:4: note: in expansion of macro ‘atomic_xchg’
  596 |    atomic_xchg(&kgdb_active, cpu);
      |    ^~~~~~~~~~~
Signed-off-by: NHelge Deller <deller@gmx.de>

arch/parisc/include/asm/cmpxchg.h

@@ -44,8 +44,14 @@ __xchg(unsigned long x, __volatile__ void *ptr, int size)
 **		if (((unsigned long)p & 0xf) == 0)
 **			return __ldcw(p);
 */
-#define xchg(ptr, x) \
-	((__typeof__(*(ptr)))__xchg((unsigned long)(x), (ptr), sizeof(*(ptr))))
+#define xchg(ptr, x)							\
+({									\
+	__typeof__(*(ptr)) __ret;					\
+	__typeof__(*(ptr)) _x_ = (x);					\
+	__ret = (__typeof__(*(ptr)))					\
+		__xchg((unsigned long)_x_, (ptr), sizeof(*(ptr)));	\
+	__ret;								\
+})
 
 /* bug catcher for when unsupported size is used - won't link */
 extern void __cmpxchg_called_with_bad_pointer(void);