Use the jobs property to specify number of threads LIT is allowed to use in LibcxxAndAbiBuilder.

Currently LIT will always use all the threads on the system and ignore the number
of jobs allowed by the slave. Fix this by adding '--threads=%(jobs)s' to the LIT
args and deduce the value using properties.


git-svn-id: https://llvm.org/svn/llvm-project/zorg/trunk@220523 91177308-0d34-0410-b5e6-96231b3b80d8