Revert r185339 (ARM: relax the atomic release barrier to "dmb ishst")

Turns out I'd misread the architecture reference manual and thought
that was a load/store-store barrier, when it's not.

Thanks for pointing it out Eli!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185356 91177308-0d34-0410-b5e6-96231b3b80d8