[CMake][runtimes] Don't process common options in runtimes build

This is no longer needed for any of the runtimes build and it breaks
in case we don't have the working compiler yet, e.g. when building
a compiler that uses compiler-rt and libc++ as a default runtime,
because these common options check whether these are available.

Differential Revision: https://reviews.llvm.org/D39932

llvm-svn: 318227