Only crosscompile binary op stubs if we compile a snapshot.

src/assembler.cc

Index: src/assembler.cc
diff --git a/src/assembler.cc b/src/assembler.cc
index cd2bedd4287ea40655c12f56a5442e51c198d826..c6228eb353eaad7da8a18f10f8c5a84c07215f3e 100644
--- a/src/assembler.cc
+++ b/src/assembler.cc
@@ -211,6 +211,9 @@ CpuFeatureScope::~CpuFeatureScope() {
 
 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;
 }