[llvm-objcopy] Fix crash for binary input files with non-ascii names

The code was using the standard isalnum function which doesn't handle
values outside the non-ascii range. Switching to using llvm::isAlnum
instead ensures we don't provoke undefined behaviour, which can in some
cases result in crashes.

Reviewed by: MaskRay

Differential Revision: https://reviews.llvm.org/D97663