[PATCH] [ARM, Fix] Fix emitLeading/TrailingFence on old ARM processors

[Robin Morisset]

Hi jfb, t.p.northover, luqmana,

I had only tested this code for ARMv7 and ARMv8. This patch adds several
fallback paths if the processor does not support dmb ish:
- dmb sy if a cortex-M with support for dmb
- mcr p15, #0, r0, c7, c10, #5 for ARMv6 (special instruction equivalent to a DMB)
- call to __sync_synchronize otherwise.
These fallback paths were chosen based on the code for fence seq_cst.

Thanks to luqmana for having noticed this bug.

http://reviews.llvm.org/D5304

Files:
  lib/Target/ARM/ARMISelLowering.cpp
  lib/Target/ARM/ARMISelLowering.h
  test/CodeGen/ARM/atomic-load-store.ll
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