Revert r324557, "gold-plugin: Do not set codegen opt level based on LTO opt level."

It was reported that this change measurably regressed -plugin-opt=O3
performance.

There is an ongoing discussion on llvm-dev about the correct way to
set the CG opt level, see thread "[llvm-dev] [RFC] Adding function
attributes to represent codegen optimization level".

llvm-svn: 329458