x86/microcode: Synchronize late microcode loading

x86/microcode: Synchronize late microcode loading

This patch ports microcode improvement patches from linux kernel.

Before you read any further: the early loading method is still the
preferred one and you should always do that. The following patch is
improving the late loading mechanism for long running jobs and cloud use
cases.

Gather all cores and serialize the microcode update on them by doing it
one-by-one to make the late update process as reliable as possible and
avoid potential issues caused by the microcode update.

Signed-off-by: Chao Gao <chao.gao@intel.com>
Tested-by: Chao Gao <chao.gao@intel.com>
[linux commit: a5321aec6412b20b5ad15db2d6b916c05349dbff]
[linux commit: bb8c13d61a629276a162c1d2b1a20a815cbcfbb7]
Reviewed-by: Jan Beulich <jbeulich@suse.com>