x86/microcode: Add loader version file in debugfs

x86/microcode: Add loader version file in debugfs

We want to be able to find out whether late microcode loader is using a
"safe" method where the system is in stop_machines() --- i.e. all cores
are pinned in kernel with interrupts disabled. This is especially
important for core siblings --- if one thread is loading microcode while
the other is executing instructions that are being patched then bad
things may happen, including MCEs.

Presense of this file indicates that we are all good. We will also
provide version value of "1".

[root@ca-virt1-0 ~]# cat /sys/kernel/debug/x86/microcode_loader_version
1
[root@ca-virt1-0 ~]#

Orabug: 29754165

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>