[MC] Change ELFOSABI_NONE to ELFOSABI_GNU for SHF_GNU_RETAIN

GNU ld does not give SHF_GNU_RETAIN GC root semantics for ELFOSABI_NONE.
(https://sourceware.org/pipermail/binutils/2021-March/115581.html)

This allows GNU ld to interpret SHF_GNU_RETAIN and avoids a gold quirk
https://sourceware.org/bugzilla/show_bug.cgi?id=27490

Because ELFObjectWriter is in an anonymous namespace, I have to place
`markGnuAbi` in the parent MCObjectWriter.

Differential Revision: https://reviews.llvm.org/D97976