[ARM] Use the load-acquire/store-release instructions optimally in AArch32.

Patch by Artyom Skrobov.

llvm-svn: 191428
diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp
index 64001b4..94069cd 100644
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp
@@ -94,7 +94,7 @@
     unsigned SubVer = TT[Idx];
     if (SubVer == '8') {
       // FIXME: Parse v8 features
-      ARMArchFeature = "+v8";
+      ARMArchFeature = "+v8,+db";
     } else if (SubVer == '7') {
       if (Len >= Idx+2 && TT[Idx+1] == 'm') {
         isThumb = true;