[CMake] Add check-compiler-rt target for runtimes build

Durning standalone builds (which includes runtimes builds) we want to create a target named check-compiler-rt. Additionally we also create check-all if it doesn't already exist as a convienence target that depends on check-compiler-rt.

This allows us to generate a single check target that invokes lit for all test suites in the runtimes projects, while avoiding name collision of check-all and not breaking existing workflows.

llvm-svn: 279334