BACKPORT: rcu/tree: Export rcu_idle_{enter,exit} to modules

Fix this link error:

  ERROR: modpost: "rcu_idle_enter" [drivers/acpi/processor.ko] undefined!
  ERROR: modpost: "rcu_idle_exit" [drivers/acpi/processor.ko] undefined!

when CONFIG_ACPI_PROCESSOR is built as module. PeterZ says that in light
of ARM needing those soon too, they should simply be exported.

Link: https://lore.kernel.org/linux-pm/20200921103741.GC5901@zn.tnic/


Bug: 172034763
Change-Id: I26d516d11edbce673f52df6474f6695c5a1e2b07
(cherry picked from commit 3ad1c8ef083bef96ec922688966484be1039e6b5)
Fixes: 1fecfdbb7acc ("ACPI: processor: Take over RCU-idle for C3-BM idle")
Reported-by: Sven Joachim <svenjoac@gmx.de>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Paul E. McKenney <paulmckrcu@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Cheng Jui Wang <cheng-jui.wang@mediatek.com>
Signed-off-by: Chun-Hung Wu <chun-hung.wu@mediatek.com>