setjmp: avoid useless REX-prefix on xor %eax, %eax

src/setjmp/x32/setjmp.s

@@ -18,5 +18,5 @@ setjmp:
 	mov %rdx,48(%rdi)
 	mov (%rsp),%rdx         /* save return addr ptr for new rip */
 	mov %rdx,56(%rdi)
-	xor %rax,%rax           /* always return 0 */
+	xor %eax,%eax           /* always return 0 */
 	ret

src/setjmp/x86_64/setjmp.s

@@ -18,5 +18,5 @@ setjmp:
 	mov %rdx,48(%rdi)
 	mov (%rsp),%rdx         /* save return addr ptr for new rip */
 	mov %rdx,56(%rdi)
-	xor %rax,%rax           /* always return 0 */
+	xor %eax,%eax           /* always return 0 */
 	ret