[CMake] Add support for exposing runtime targets

This patch adds support to the runtimes build for exposing sub-project targets through the high-level configuration. This will enable exposing the build, check and install targets for sub-project components (i.e. asan, check-asan, install-asan...).

This patch requires minor changes to the runtime projects to take advantage of it, and I'll phase those changes into Compiler-RT shortly.

llvm-svn: 279776