[CMake] Propagate top-level targets for compiler-rt runtimes and test-suites

from (external) compiler-rt build tree into LLVM/Clang build tree in
LLVM_BUILD_EXTERNAL_COMPILER_RT mode.

For instance, running
  make asan -j12
in LLVM/Clang build tree will now build Clang, use it to configure
compiler-rt build tree, and invoke "make asan -j12" there. ASan runtime will
be built in the proper location, where Clang driver expects to find it.

Running
  make check-asan
will build Clang, use it to configure compiler-rt build tree, build everything
there, and then run "make check-asan" in compiler-rt build tree using just-built
Clang and ASan runtime.

llvm-svn: 204463