I have a Celeron 847 platform (x86_64) that works fine on kernels <= 4.14.33
But a regression was introduced into 4.14.34 that causes a PANIC and reboot when system initialization executes this:

  echo 1 > /sys/devices/system/cpu/microcode/reload

That line is in rc.local, and is used during boot to update to the latest microcode (from the intel-ucode repository).

Bisecting to find the culprit, we have this:
    commit 8413a3a63d3717504f2db5d6c3b018cabf15d132
    Author: Borislav Petkov <bp@suse.de>
    Date:   Wed Mar 14 19:36:15 2018 +0100
        x86/microcode: Fix CPU synchronization routine
        commit bb8c13d61a629276a162c1d2b1a20a815cbcfbb7 upstream.

Reverting that patch (or simply not performing the microcode update) makes this Celeron-847 happy again.

Here is some console output, thanks to netconsole:
    microcode: Timeout while waiting for CPUs rendezvous, remaining: 1
    Kernel panic - not syncing: Timeout during microcode update!
    CPU: 0 PID: 10 Comm: migration/0 Not tainted 4.14.33+ #1
    Hardware name: ECS NM70-TI/NM70-TI, BIOS 4.6.5 05/30/2013
    Call Trace:
      dump_stack+0x46/0x68
      panic+0xca/0x208
      __reload_late+0xab/0xb0
      multi_cpu_stop+0x4f/0xa0
      ? cpu_stop_queue_work+0x70/0x70
      cpu_stopper_thread+0x7f/0xf0
      ? sort_range+0x20/0x20
      smpboot_thread_fn+0xda/0x150
      kthread+0x105/0x120
      ? __kthread_parkme+0x70/0x70
      ? __kthread_parkme+0x70/0x70
      ret_from_fork+0x1f/0x30
      Shutting down cpus with NMI

ObBugzilla:  Sorry for filing this in "platform", but I couldn't find a bugzilla "product" for CPU.  I toyed with "device drivers" but didn't see anything relevant.