Apply fix for bug#45131, "Problems building on 32-bit linux".

As described in the bug, the problem is not new in 5.4, but newly tickled.
We need a register whose lower 8 bits can be accessed, and must request
such from the assembler.  Using a "q" constraint instead of "r" constraint
accomplishes this.

include/atomic/x86-gcc.h

@@ -35,7 +35,7 @@
   asm volatile (LOCK "; xadd %0, %1;" : "+r" (v) , "+m" (*a))
 #endif
 #define make_atomic_swap_body(S)				\
-  asm volatile ("; xchg %0, %1;" : "+r" (v) , "+m" (*a))
+  asm volatile ("; xchg %0, %1;" : "+q" (v) , "+m" (*a))
 #define make_atomic_cas_body(S)					\
   asm volatile (LOCK "; cmpxchg %3, %0; setz %2;"		\
                : "+m" (*a), "+a" (*cmp), "=q" (ret): "r" (set))