Some users of xchg() don't bother using the return value, which results
in a compiler warning like the following (from kgdb):

In file included from linux/arch/arm64/include/asm/atomic.h:27:0,
                 from include/linux/atomic.h:4,
                 from include/linux/spinlock.h:402,
                 from include/linux/seqlock.h:35,
                 from include/linux/time.h:5,
                 from include/uapi/linux/timex.h:56,
                 from include/linux/timex.h:56,
                 from include/linux/sched.h:19,
                 from include/linux/pid_namespace.h:4,
                 from kernel/debug/debug_core.c:30:
kernel/debug/debug_core.c: In function ‘kgdb_cpu_enter’:
linux/arch/arm64/include/asm/cmpxchg.h:75:3: warning: value computed is not used [-Wunused-value]
  ((__typeof__(*(ptr)))__xchg((unsigned long)(x),(ptr),sizeof(*(ptr))))
   ^
linux/arch/arm64/include/asm/atomic.h:132:30: note: in expansion of macro ‘xchg’
 #define atomic_xchg(v, new) (xchg(&((v)->counter), new))

kernel/debug/debug_core.c:504:4: note: in expansion of macro ‘atomic_xchg’
    atomic_xchg(&kgdb_active, cpu);
    ^

This patch makes use of the same trick as we do for cmpxchg, by assigning
the return value to a dummy variable in the xchg() macro itself.
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>