PPC: Add a MathFloor stub generated with TurboFan

Port abc35080

Original commit message:
This stub will be used as the basis of a Math.floor-specific CallIC to
detect and track calls to floor that return -0.

Along the way:
- Create a TurboFanCodeStub super class from which the StringLength and
MathRound TF stubs derive.
- Fix the ugly hack that passes the first stub parameter as the "this"
pointer in the the TF-compiled JS function.
- Fix bugs in the ia32/x64 disassembler.

R=danno@chromium.org, dstence@us.ibm.com, michael_dawson@ca.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1137033002

Cr-Commit-Position: refs/heads/master@{#28341}

src/ppc/interface-descriptors-ppc.cc

@@ -369,6 +369,21 @@ void ApiAccessorDescriptor::Initialize(CallInterfaceDescriptorData* data) {
   };
   data->Initialize(arraysize(registers), registers, representations);
 }
+
+
+void MathRoundVariantDescriptor::Initialize(CallInterfaceDescriptorData* data) {
+  Register registers[] = {
+      cp,  // context
+      r4,  // math rounding function
+      r6,  // vector slot id
+  };
+  Representation representations[] = {
+      Representation::Tagged(),  //
+      Representation::Tagged(),  //
+      Representation::Tagged(),  //
+  };
+  data->Initialize(arraysize(registers), registers, representations);
+}
 }
 }  // namespace v8::internal