x86/cpu: Provide the full setup for getcpu() on x86-32

setup_getcpu() configures two things:

  - it writes the current CPU & node information into MSR_TSC_AUX
  - it writes the same information as a GDT entry.

By using the "full" setup_getcpu() on i386 it is possible to read the CPU
information in userland via RDTSCP() or via LSL from the GDT.

Provide an GDT_ENTRY_CPUNODE for x86-32 and make the setup function
unconditionally available.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Roland Mainz <roland.mainz@nrubsig.org>
Link: https://lore.kernel.org/r/20221125094216.3663444-2-bigeasy@linutronix.de