[Compiler-rt][Builtins] Implement lit-test support (part 2 of 2)

Summary:
Original r297566 (https://reviews.llvm.org/D30802) is splitted into two parts.
This part adds CMakefile/lit.cfg support.

Reviewers: rengolin, compnerd, jroelofs, erik.pilkington

Subscribers: srhines, dberris, mgorny

Differential Revision: https://reviews.llvm.org/D31259

llvm-svn: 298714