Revert of [builtins] Introduce proper Float64Exp operator.

src/runtime/runtime-maths.cc

Index: src/runtime/runtime-maths.cc
diff --git a/src/runtime/runtime-maths.cc b/src/runtime/runtime-maths.cc
index fb8e083debff2cca7f0ad648aa9d49d5dadfdb72..47fe705400874f062ffee72541194ab3be4187d5 100644
--- a/src/runtime/runtime-maths.cc
+++ b/src/runtime/runtime-maths.cc
@@ -56,6 +56,17 @@
   FixedFloat64Array* array = FixedFloat64Array::cast(result->elements());
   double* y = static_cast<double*>(array->DataPtr());
   return Smi::FromInt(fdlibm::rempio2(x, y));
+}
+
+
+RUNTIME_FUNCTION(Runtime_MathExpRT) {
+  HandleScope scope(isolate);
+  DCHECK(args.length() == 1);
+  isolate->counters()->math_exp_runtime()->Increment();
+
+  CONVERT_DOUBLE_ARG_CHECKED(x, 0);
+  lazily_initialize_fast_exp(isolate);
+  return *isolate->factory()->NewNumber(fast_exp(x, isolate));
 }