Use ninja pools to limit the number of concurrent compile/link jobs.

This change makes use of the new "job pool" capability in cmake 3.0
with ninja generator to allow limiting the number of concurrent jobs
of a certain type.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222341 91177308-0d34-0410-b5e6-96231b3b80d8