[CMake] NFC. Add support for testing the compiler without testing the linker

Summary:
One of the big limitations we have in the compiler-rt build system today is that we cannot bootstrap building the builtins because you need a fully functional toolchain to pass CMake's tests.

This change adds support for compile only tests.

It is NFC because nothing is using the compile-only tests yet.

I believe this is the last separable part of D16653.

Reviewers: samsonov

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D19692

llvm-svn: 268427