Add a tool to split Intel microcode into one file per Platform Id

Intel now releases microcode updates in files named after
<family>-<model>-<stepping>. In some cases a single file may include
microcode for multiple Platform Ids for the same family, model, and
stepping. Our current microcode update tooling (/usr/sbin/cpucontrol)
only processes the first microcode update in the file.

This tool splits combined files into individual files with one microcode
update each, named as
<family>-<model>-<stepping>.<platform_id_mask>.

Adding this to tools/ for experimentation and testing. In the future
we'll want to have cpucontrol or other tooling work directly with the
Intel-provided microcode files.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D15433