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

Summary:
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)
These fallback paths were chosen based on the code for fence seq_cst.

Thanks to luqmana for having noticed this bug.

Test Plan: Added more cases to atomic-load-store.ll + make check-all

Reviewers: jfb, t.p.northover, luqmana

Subscribers: aemerson, llvm-commits

Differential Revision: http://reviews.llvm.org/D5304