[gold] Split plugin options controlling ThinLTO and codegen parallelism.

Summary:
As suggested in D24826, use different options for ThinLTO backend
parallelism from the option controlling regular LTO code gen
parallelism. They are already split in the LTO API, and this enables
controlling them with different clang options.

Reviewers: pcc, mehdi_amini

Subscribers: dexonsmith, llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D24873

llvm-svn: 282290