[llvm-readobj] fix unit test failure on 32bit machines

Several bots are failing on 32-bit since https://reviews.llvm.org/D145761 was merged
  https://lab.llvm.org/buildbot/#/builders/178/builds/4384

It seems due to the use of uintptr_t (32bit here) for storing 64 bit values.

Issue is fixed by replacing to uint64_t (as suggested by DavidSpickett).

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D148208