[LLVM][Runtimes] Hide unused CMake variable messages for runtimes builds

The LLVM runtime build is used to bootstrap projects with the built LLVM
toolchain. This effectively re-runs CMake with the current build
directory. One problem is that this passes every common CMake variable
to the projects individually, some of which are not necessarily used.

This patch suppresses the unused variable warnings for the runtimes.
The standard CMake invocation should still be able to print out the
unused variables so it should not impact code quality.

Reviewed By: thieta

Differential Revision: https://reviews.llvm.org/D143199