Fix build problems on Windows 64-bit by casting.

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 b5bb6365d3c8deb8f07f640e392ac485214c23e7..dabb764a9ff0f1c22f0e54bf52ae65ccd67a7147 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -37,6 +37,7 @@ namespace internal {
 // a spare register). The register isn't callee save, and not used by the
 // function calling convention.
 static const Register kScratchRegister = { 10 };  // r10.
+static const Register kRootRegister = { 13 };     // r13
 
 // Convenience for platform-independent signatures.
 typedef Operand MemOperand;