[Offload] Fix missing abs function for test

Summary:
We don't have the abs function to link against, just use the builtin.

offload/test/offloading/ompx_bare_shfl_down_sync.cpp

@@ -23,7 +23,7 @@ bool equal(T LHS, T RHS) {
 template <typename T,
           std::enable_if_t<std::is_floating_point<T>::value, bool> = true>
 bool equal(T LHS, T RHS) {
-  return std::abs(LHS - RHS) < std::numeric_limits<T>::epsilon();
+  return __builtin_fabs(LHS - RHS) < std::numeric_limits<T>::epsilon();
 }
 
 template <typename T> void test() {