limit to 2 parallel links when using thinlto

Summary:
When using ThinLTO, the linker performs its own parallelism. This
change limits the number of parallel link jobs that Ninja will issue
to keep the total number of threads reasonable when linking with
ThinLTO.

Reviewers: hans, ruiu

Subscribers: mgorny, mehdi_amini, Prazek

Differential Revision: https://reviews.llvm.org/D31990

llvm-svn: 301676