[RegisterConfiguration] Streamline access to arch defaults, simplify Registers.

test/cctest/test-code-stubs-x64.cc

Index: test/cctest/test-code-stubs-x64.cc
diff --git a/test/cctest/test-code-stubs-x64.cc b/test/cctest/test-code-stubs-x64.cc
index 786da547e4ec4b92c5d2e74c0635c59e0e2b1fc2..b9eddd6c57fff17b0505977d6eef73999f8628b7 100644
--- a/test/cctest/test-code-stubs-x64.cc
+++ b/test/cctest/test-code-stubs-x64.cc
@@ -64,9 +64,7 @@ ConvertDToIFunc MakeConvertDToIFuncTrampoline(Isolate* isolate,
   __ pushq(rsi);
   __ pushq(rdi);
 
-
-  const RegisterConfiguration* config =
-      RegisterConfiguration::ArchDefault(RegisterConfiguration::CRANKSHAFT);
+  const RegisterConfiguration* config = RegisterConfiguration::Crankshaft();
   if (!source_reg.is(rsp)) {
     // The argument we pass to the stub is not a heap number, but instead
     // stack-allocated and offset-wise made to look like a heap number for