[CMake][runtimes] Set compiler as working even for default target

Even when building builtins and runtimes for the default target
we shouldn't assume that the just built compiler is already useable.
When the compiler uses compiler-rt and libc++ as the default runtime
and C++ library, it won't be usable until we finish building runtimes.

Differential Revision: https://reviews.llvm.org/D39715

llvm-svn: 318224