sparc: make xchg() into a statement expression

Fix several of the same warning by using a GCC statement expression
(extension):

../arch/sparc/include/asm/cmpxchg_32.h:28:22: warning: value computed is not used [-Wunused-value]
   28 | #define xchg(ptr,x) ((__typeof__(*(ptr)))__xchg((unsigned long)(x),(ptr),sizeof(*(ptr))))

Passes sparc32 allmodconfig with no xchg warnings.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: sparclinux@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>