[llvm-readobj] Prepend argv[0] to error/warning messages

Summary:
Currently, we report:

    error: ...

Prepend argv[0] (tool name):

    llvm-readobj: error: ...

This is consistent with most GNU binutils/clang/lld, and gives a bit
more context in a long build log.

Reviewed By: grimar, jhenderson, rupprecht

Differential Revision: https://reviews.llvm.org/D66425

llvm-svn: 369377