[x64] Make xmm0 allocatable and use xmm15 as scratch register instead.

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index 013d0f13b5dc391e6f08e78dac277a27a2ee88cd..b088c7d19c44a1d2301af08c19fb2f689b56df2c 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -34,8 +34,9 @@ const Register kRuntimeCallArgCountRegister = {Register::kCode_rax};
 // Default scratch register used by MacroAssembler (and other code that needs
 // a spare register). The register isn't callee save, and not used by the
 // function calling convention.
-const Register kScratchRegister = { 10 };      // r10.
-const Register kRootRegister = { 13 };         // r13 (callee save).
+const Register kScratchRegister = {10};      // r10.
+const XMMRegister kScratchDoubleReg = {15};  // xmm15.
+const Register kRootRegister = {13};         // r13 (callee save).
 // Actual value of root register is offset from the root array's start
 // to take advantage of negitive 8-bit displacement values.
 const int kRootRegisterBias = 128;