x86/entry/32: Clarify register saving in __switch_to_asm()

commit 6690e86b ("sched/x86: Save [ER]FLAGS on context switch")
re-introduced the flags saving on context switch to prevent AC leakage.

The pushf/popf instructions are right among the callee saved register
section, so the comment explaining the save/restore is not entirely
correct.

Add a seperate comment to pushf/popf explaining the reason.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>