mc146818rtc: ensure LOST_TICK_POLICY_SLEW is only enabled on TARGET_I386

mc146818rtc: ensure LOST_TICK_POLICY_SLEW is only enabled on TARGET_I386

Any tick policy specified on other platforms rather on TARGET_I386
will fall back to LOST_TICK_POLICY_DISCARD silently, this patch makes
sure only TARGET_I386 can enable LOST_TICK_POLICY_SLEW

After that, we can enable LOST_TICK_POLICY_SLEW in the common code
which need not use '#ifdef TARGET_I386' to make these code be x86
specific anymore

Signed-off-by: Xiao Guangrong <xiaoguangrong@tencent.com>
Message-Id: <20170510083259.3900-4-xiaoguangrong@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>