fix i386 __set_thread_area fallback

this code is only needed for pre-2.6 kernels, which are not actually
supported anyway, and was never tested. the fallback path using
SYS_modify_ldt failed to clear the upper bits of %eax (all ones due to
SYS_set_thread_area's return value being an error) before modifying
%al to attempt a new syscall.

src/thread/i386/__set_thread_area.s

@@ -28,6 +28,7 @@ __set_thread_area:
 	ret
 2:
 	mov %ebx,%ecx
+	xor %eax,%eax
 	xor %ebx,%ebx
 	xor %edx,%edx
 	mov %ebx,(%esp)