x86/microcode: Update late microcode in parallel

Microcode update was changed to be serialized due to restrictions after
Spectre days. Updating serially on a large multi-socket system can be
painful since it is being done on one CPU at a time.

Cloud customers have expressed discontent as services disappear for
a prolonged time. The restriction is that only one core (or only one
thread of a core in the case of an SMT system) goes through the update
while other cores (or respectively, SMT threads) are quiesced.

Do the microcode update only on the first thread of each core while
other siblings simply wait for this to complete.

 [ bp: Simplify, massage, cleanup comments. ]

Signed-off-by: Ashok Raj <[email protected]>
Signed-off-by: Mihai Carabas <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jon Grimm <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: Thomas Gleixner <[email protected]>
Cc: Tom Lendacky <[email protected]>
Cc: x86-ml <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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