[CMake] [Runtimes] Set *_STANDALONE_BUILD

Summary:
The runtimes use `*_STANDALONE_BUILD=OFF` to signify that clang is an in-tree target. This is not the case with the runtime builds, so we really need this set to `ON`.

In order to resolve the issues phosek was having with checks, we should use checks that don't link. We can use compiler-rt's `try_compile_only` as a basis for that.

This patch is *required* to be able to run the runtime libraries check-* targets.

Reviewers: smeenai, phosek, compnerd

Reviewed By: phosek

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62410

llvm-svn: 362007