[cmake] Make it possible to build and test profile without sanitizers

This should fix an issue which arises when running check-compiler-rt on
the coverage bot:
http://green.lab.llvm.org/green/job/clang-stage2-coverage-R_build/1590/

The bot doesn't build the sanitizers, but the check-compiler-rt target
always expects the profile runtime to exist.

llvm-svn: 313549