[ELF] - LTO: Try to be option compatible with the gold plugin.

This is relative to PR30720.

Previously we ignored all --plugin-opt arguments.
Patch adds support for them.

Patch does not add any new LTO options,
and just implements mapping from --plugin-opt to existent ones.

Differential revision: https://reviews.llvm.org/D36227

llvm-svn: 310826