[yaml2obj][obj2yaml] - Teach yaml2obj/obj2yaml tools about STB_GNU_UNIQUE symbols.

yaml2obj/obj2yaml does not support the symbols with STB_GNU_UNIQUE yet.
Currently, obj2yaml fails with llvm_unreachable when met such a symbol.

I faced it when investigated the https://bugs.llvm.org/show_bug.cgi?id=41196.

Differential revision: https://reviews.llvm.org/D59875

llvm-svn: 357158