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

src/IceTargetLoweringX86BaseImpl.h

Index: src/IceTargetLoweringX86BaseImpl.h
diff --git a/src/IceTargetLoweringX86BaseImpl.h b/src/IceTargetLoweringX86BaseImpl.h
index 729bc919d7f1728efebe2a1d2a50d925a84d2279..71460d5f961cfb8aa73b2dd02dd7a79aeb8f3570 100644
--- a/src/IceTargetLoweringX86BaseImpl.h
+++ b/src/IceTargetLoweringX86BaseImpl.h
@@ -5511,7 +5511,7 @@ 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