[yaml2obj/obj2yaml/llvm-readobj] Support SHF_GNU_RETAIN

In binutils, the flag is defined for ELFOSABI_GNU and ELFOSABI_FREEBSD.
It can be used to mark a section as a GC root.

In practice, the flag has generic semantics and can be applied to many
EI_OSABI values, so we consider it generic.

Differential Revision: https://reviews.llvm.org/D95728