PPC: [Proxies] Support constructable proxy as new.target (reland)

Port 7ceaf727

R=verwaest@chromium.org, joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=v8:1543, v8:3330, v8:3931
LOG=n

Review URL: https://codereview.chromium.org/1480423002

Cr-Commit-Position: refs/heads/master@{#32433}

src/ppc/builtins-ppc.cc

@@ -1776,10 +1776,9 @@ void Builtins::Generate_ConstructFunction(MacroAssembler* masm) {
   // ----------- S t a t e -------------
   //  -- r3 : the number of arguments (not including the receiver)
   //  -- r4 : the constructor to call (checked to be a JSFunction)
-  //  -- r6 : the new target (checked to be a JSFunction)
+  //  -- r6 : the new target (checked to be a constructor)
   // -----------------------------------
   __ AssertFunction(r4);
-  __ AssertFunction(r6);
 
   // Calling convention for function specific ConstructStubs require
   // r5 to contain either an AllocationSite or undefined.