X87: implement MathPow stub for X87.

  In CL 0fe2fbd1 the implementation of
  MathPow for all ports are unified and MathPow stub code is invoked.
  So we move the direct runtime function call from full-codegen to MathPow
  stub for X87 platform.

BUG=

Review URL: https://codereview.chromium.org/1258873002

Cr-Commit-Position: refs/heads/master@{#29865}

src/x87/code-stubs-x87.cc

@@ -325,8 +325,28 @@ void FloatingPointHelper::CheckFloatOperands(MacroAssembler* masm,
 
 
 void MathPowStub::Generate(MacroAssembler* masm) {
-  // No SSE2 support
-  UNREACHABLE();
+  const Register base = edx;
+  const Register scratch = ecx;
+  Counters* counters = isolate()->counters();
+  Label call_runtime;
+
+  // We will call runtime helper function directly.
+  if (exponent_type() == ON_STACK) {
+    // The arguments are still on the stack.
+    __ bind(&call_runtime);
+    __ TailCallRuntime(Runtime::kMathPowRT, 2, 1);
+
+    // The stub is called from non-optimized code, which expects the result
+    // as heap number in exponent.
+    __ AllocateHeapNumber(eax, scratch, base, &call_runtime);
+    __ fstp_d(FieldOperand(eax, HeapNumber::kValueOffset));
+    __ IncrementCounter(counters->math_pow(), 1);
+    __ ret(2 * kPointerSize);
+  } else {
+    // Currently it's only called from full-compiler and exponent type is
+    // ON_STACK.
+    UNIMPLEMENTED();
+  }
 }