[cmake] Add a separate CMake var to control profile runtime

Make it possible to control building profile runtime separately from
other options. Before r313549, the profile runtime building was
controlled along with sanitizers. However, since that commit it is built
unconditionally which results in multiple builds for people building
different runtimes separately.

Differential Revision: https://reviews.llvm.org/D38441

llvm-svn: 314646