PPC/s390: [builtins] Introduce proper Float64Exp operator.

src/crankshaft/s390/lithium-codegen-s390.cc

Index: src/crankshaft/s390/lithium-codegen-s390.cc
diff --git a/src/crankshaft/s390/lithium-codegen-s390.cc b/src/crankshaft/s390/lithium-codegen-s390.cc
index 7ff094c2f452c69b71fb97b597b0a9262ad0cc16..ee02f0b31265f16d4750d9646abbc4ea16c7eef0 100644
--- a/src/crankshaft/s390/lithium-codegen-s390.cc
+++ b/src/crankshaft/s390/lithium-codegen-s390.cc
@@ -3670,15 +3670,10 @@ void LCodeGen::DoPower(LPower* instr) {
 }
 
 void LCodeGen::DoMathExp(LMathExp* instr) {
-  DoubleRegister input = ToDoubleRegister(instr->value());
-  DoubleRegister result = ToDoubleRegister(instr->result());
-  DoubleRegister double_scratch1 = ToDoubleRegister(instr->double_temp());
-  DoubleRegister double_scratch2 = double_scratch0();
-  Register temp1 = ToRegister(instr->temp1());
-  Register temp2 = ToRegister(instr->temp2());
-
-  MathExpGenerator::EmitMathExp(masm(), input, result, double_scratch1,
-                                double_scratch2, temp1, temp2, scratch0());
+  __ PrepareCallCFunction(0, 1, scratch0());
+  __ MovToFloatParameter(ToDoubleRegister(instr->value()));
+  __ CallCFunction(ExternalReference::ieee754_exp_function(isolate()), 0, 1);
+  __ MovFromFloatResult(ToDoubleRegister(instr->result()));
 }
 
 void LCodeGen::DoMathLog(LMathLog* instr) {