[CMake] Provide options for toggling on and off various runtime libraries.

Summary:
Rather than having to add new "experimental" options each time someone wants to work on bringing a sanitizer to a new platform, this patch makes options for all of them.

The default values for the options are set by the platform checks that would have enabled them, but they can be overridden on or off.

Reviewers: kubabrecka, samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14846

llvm-svn: 255170