xtensa: implement fake NMI

In case perf IRQ is the highest of the medium-level IRQs, and is alone
on its level, it may be treated as NMI:
- LOCKLEVEL is defined to be one level less than EXCM level,
- IRQ masking never lowers current IRQ level,
- new fake exception cause code, EXCCAUSE_MAPPED_NMI is assigned to that
  IRQ; new second level exception handler, do_nmi, assigned to it
  handles it as NMI,
- atomic operations in configurations without s32c1i still need to mask
  all interrupts.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

diff --cc arch/xtensa/include/asm/atomic.h
Simple merge

diff --cc arch/xtensa/include/asm/cmpxchg.h
Simple merge

diff --cc arch/xtensa/include/asm/irqflags.h
Simple merge

diff --cc arch/xtensa/include/asm/processor.h
Simple merge

diff --cc arch/xtensa/kernel/entry.S
Simple merge

diff --cc arch/xtensa/kernel/irq.c
Simple merge

diff --cc arch/xtensa/kernel/perf_event.c
Simple merge

diff --cc arch/xtensa/kernel/traps.c
Simple merge

diff --cc arch/xtensa/kernel/vectors.S
Simple merge