Fix building with clang

src/arm/assembler-arm.h

 // Copyright (c) 1994-2006 Sun Microsystems Inc.
 // All Rights Reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions
 // are met:
 //
 // - Redistributions of source code must retain the above copyright notice,
 // this list of conditions and the following disclaimer.
 //
@@ skipping 282 matching lines @@
 const DwVfpRegister d7  = {  7 };
 const DwVfpRegister d8  = {  8 };
 const DwVfpRegister d9  = {  9 };
 const DwVfpRegister d10 = { 10 };
 const DwVfpRegister d11 = { 11 };
 const DwVfpRegister d12 = { 12 };
 const DwVfpRegister d13 = { 13 };
 const DwVfpRegister d14 = { 14 };
 const DwVfpRegister d15 = { 15 };
 
-// Aliases for double registers.
-static const DwVfpRegister& kFirstCalleeSavedDoubleReg = d8;
-static const DwVfpRegister& kLastCalleeSavedDoubleReg = d15;
-static const DwVfpRegister& kDoubleRegZero = d14;
-static const DwVfpRegister& kScratchDoubleReg = d15;
+// Aliases for double registers.  Defined using #define instead of
+// "static const DwVfpRegister&" because Clang complains otherwise when a
+// compilation unit that includes this header doesn't use the variables.
+#define kFirstCalleeSavedDoubleReg d8
+#define kLastCalleeSavedDoubleReg d15
+#define kDoubleRegZero d14
+#define kScratchDoubleReg d15
 
 
 // Coprocessor register
 struct CRegister {
   bool is_valid() const { return 0 <= code_ && code_ < 16; }
   bool is(CRegister creg) const { return code_ == creg.code_; }
   int code() const {
     ASSERT(is_valid());
     return code_;
   }
@@ skipping 1093 matching lines @@
  public:
   explicit EnsureSpace(Assembler* assembler) {
     assembler->CheckBuffer();
   }
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_ARM_ASSEMBLER_ARM_H_