Cherry-pick r244675: [MIPS] Use arch values for lock-free atomic ops

Original commit message:

[MIPS] Use arch values for lock-free atomic operations

Let NaClMips32ELTargetInfo inherit arch values for maximum width lock-free
atomic operations.

Differential Revision: http://reviews.llvm.org/D11949

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4205
R=dschuff@chromium.org

Review URL: https://codereview.chromium.org/1275423007 .
diff --git a/lib/Basic/Targets.cpp b/lib/Basic/Targets.cpp
index 5c0fa42..f8a2882 100644
--- a/lib/Basic/Targets.cpp
+++ b/lib/Basic/Targets.cpp
@@ -6569,8 +6569,7 @@
 class NaClMips32ELTargetInfo : public Mips32ELTargetInfo {
 public:
   NaClMips32ELTargetInfo(const llvm::Triple &Triple) :
-    Mips32ELTargetInfo(Triple)  {
-      MaxAtomicPromoteWidth = MaxAtomicInlineWidth = 0;
+    Mips32ELTargetInfo(Triple) {
   }
 
   BuiltinVaListKind getBuiltinVaListKind() const override {