ARM: relax the atomic release barrier to "dmb ishst"

I believe the full "dmb ish" barrier is not required to guarantee release
semantics for atomic operations. The weaker "dmb ishst" prevents previous
operations being reordered with a store executed afterwards, which is enough.

A key point to note (fortunately already correct) is that this barrier alone is
*insufficient* for sequential consistency, no matter how liberally placed.

llvm-svn: 185339