selftests: Emit a warning if getcpu() is missing on 32bit

The VDSO implementation for getcpu() has been wired up on 32bit so warn if
missing.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20221125094216.3663444-4-bigeasy@linutronix.de