[llvm-build] Force lit tests to run in parallel

This is achieved by setting the CMake option LLVM_LIT_ARGS. If we don't do this,
lit may decide that it only needs to run on one core.

While we're at it, also update the run.sh that we use for bisects to force ninja
to use the right number of cores while building.

Change-Id: I099697f4a377541f907d8f13f8f807c2a4f901d6
3 files changed