[llvm-readobj] Avoid use of std::abs to fix ubsan failure.

[craig.topper]

Summary

The input here can be INT64_MIN. We already know the number is
negative on the path that used std::abs. Cast to uint64_t and
negate that.

[MaskRay]

Oh, I did this just now. Fixed by 07bb66784c17ef6b98ce38f9cc74d1d92f1aac08

llvm/tools/llvm-readobj/ELFDumper.cpp

 //===- ELFDumper.cpp - ELF-specific dumper --------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 ///
for (const Field &F : Fields)
     printField(F);
 
   std::string Addend;
   if (std::optional<int64_t> A = R.Addend) {
     int64_t RelAddend = *A;
     if (!Fields[4].Str.empty()) {
       if (RelAddend < 0) {
         Addend = " - ";
-        RelAddend = std::abs(RelAddend);
+        RelAddend = -(uint64_t)RelAddend;
       } else {
         Addend = " + ";
       }
     }
     Addend += utohexstr(RelAddend, /*LowerCase=*/true);
   }
   OS << Addend << "\n";
 }