x86/microcode: Use native CPUID to tickle out microcode revision

x86/microcode: Use native CPUID to tickle out microcode revision

Intel supplies the microcode revision value in MSR 0x8b
(IA32_BIOS_SIGN_ID) after CPUID(1) has been executed. Execute it each
time before reading that MSR.

It used to do sync_core() which did do CPUID but

  60fe7974eb74 ("x86/asm: Rewrite sync_core() to use IRET-to-self")

changed the sync_core() implementation so we better make the microcode
loading case explicit, as the SDM documents it.

Reported-and-tested-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/20170109114147.5082-3-bp@alien8.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>