[llvm-readobj] Don't print '@' at end of unversioned dynsym names

This fixes https://bugs.llvm.org/show_bug.cgi?id=40097. The problem was
caused by a regression in r188022.

See also r350614.

Reviewed by: rupprecht, mstorsjo, Higuoxing, jakehehrlich

Differential Revision: https://reviews.llvm.org/D56319

llvm-svn: 350615