[gold] emit assembly listing from gold plugin on LTO stage

Summary:
Sometimes it's useful to emit assembly after LTO stage to modify it manually. Emitting precodegen bitcode file (via save-temps plugin option) and then feeding it to llc doesn't always give the same binary as original.
This patch is simpler alternative to https://reviews.llvm.org/D24020.

Patch by Denis Bakhvalov.

Reviewers: mehdi_amini, tejohnson

Reviewed By: tejohnson

Subscribers: MaskRay, inglorion, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D56114

llvm-svn: 350276