[LTO] Add -flto-jobs=N to control backend parallelism

Summary:
Currently, a linker option must be used to control the backend
parallelism of ThinLTO. The linker option varies depending on the
linker (e.g. gold vs ld64). Add a new clang option -flto-jobs=N
to control this.

I've added in the wiring to pass this to the gold plugin. I also
added in the logic to pass this down in the form I understand that
ld64 uses on MacOS, for the darwin target.

Reviewers: mehdi_amini, dexonsmith

Subscribers: mehdi_amini, cfe-commits

Differential Revision: https://reviews.llvm.org/D24826

llvm-svn: 282291