Merged r17130 into trunk branch.

Only crosscompile binary op stubs if we compile a snapshot.

R=verwaest@chromium.org
BUG=

Review URL: https://codereview.chromium.org/26279012

git-svn-id: http://v8.googlecode.com/svn/trunk@17132 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
diff --git a/src/assembler.cc b/src/assembler.cc
index cd2bedd..c6228eb 100644
--- a/src/assembler.cc
+++ b/src/assembler.cc
@@ -211,6 +211,9 @@
 
 PlatformFeatureScope::PlatformFeatureScope(CpuFeature f)
     : old_cross_compile_(CpuFeatures::cross_compile_) {
+  // CpuFeatures is a global singleton, therefore this is only safe in
+  // single threaded code.
+  ASSERT(Serializer::enabled());
   uint64_t mask = static_cast<uint64_t>(1) << f;
   CpuFeatures::cross_compile_ |= mask;
 }