Subzero: Fix build warnings/errors under g++.

src/IceTargetLoweringX86BaseImpl.h

 //===- subzero/src/IceTargetLoweringX86BaseImpl.h - x86 lowering -*- C++ -*-==//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 5493 matching lines @@
     _mov(Reg, Src);
   }
   return Reg;
 }
 
 namespace {
 
 template <typename T> bool isPositiveZero(T Val) {
   static_assert(std::is_floating_point<T>::value,
                 "Input type must be floating point");
-  return Val == 0 && !signbit(Val);
+  return Val == 0 && !std::signbit(Val);
 }
 
 } // end of anonymous namespace
 
 template <class Machine>
 Operand *TargetX86Base<Machine>::legalize(Operand *From, LegalMask Allowed,
                                           int32_t RegNum) {
   Type Ty = From->getType();
   // Assert that a physical register is allowed. To date, all calls to
   // legalize() allow a physical register. If a physical register needs to be
@@ skipping 505 matching lines @@
   }
   // the offset is not eligible for blinding or pooling, return the original
   // mem operand
   return MemOperand;
 }
 
 } // end of namespace X86Internal
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICETARGETLOWERINGX86BASEIMPL_H