ucode: use wrmsr_early_safe() for early CPU microcode update

Use the new method for wrmsr that is able to set up fault handler even at
the very early stage of kernel boot. This prevents panic with new
microcode for some new AMD EPYCs, that requires certain patchlevel of
microcode to be already present in the CPU. If BSP patching failed, we
print the message and we don't try to patch APs.

This has been tested only on AMD booted in EFI mode.

Reviewed by: stas, kib
Differential Revision: https://reviews.freebsd.org/D52643