[llvm-readelf] - A fix for: "--hash-symbols asserts for 64-bit ELFs"

Fixes https://bugs.llvm.org/show_bug.cgi?id=42622.
(--hash-symbols switch is currently broken for 64-bit ELF files, due to r352630.)

Differential revision: https://reviews.llvm.org/D64788

llvm-svn: 366558