cpuctl: run amd_ucode_wrmsr only on one CPU and report if it failed

The CPUCTL_UPDATE is supposed to be applied only to the CPU the ioctl(2)
was performed on. This is true for Intel CPUs, but for AMD the SMP
rendezvouz of amd_ucode_wrmsr() effectively executed it on all CPUs.
Also, the update failure was not reported.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D52466