x86/cpu: Init AP exception handling from cpu_init_secondary()

SEV-ES guests require properly setup task register with which the TSS
descriptor in the GDT can be located so that the IST-type #VC exception
handler which they need to function properly, can be executed.

This setup needs to happen before attempting to load microcode in
ucode_cpu_init() on secondary CPUs which can cause such #VC exceptions.

Simplify the machinery by running that exception setup from a new function
cpu_init_secondary() and explicitly call cpu_init_exception_handling() for
the boot CPU before cpu_init(). The latter prepares for fixing and
simplifying the exception/IST setup on the boot CPU.

There should be no functional changes resulting from this patch.

[ tglx: Reworked it so cpu_init_exception_handling() stays seperate ]

Signed-off-by: Borislav Petkov <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Lai Jiangshan <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>                                                                                                                                                                                                                        
Link: https://lore.kernel.org/r/[email protected]

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