[CMake] Change the flag to use compiler-rt builtins to boolean

This changes the name and the type to what it was prior to r333037
which matches the name of the flag used in other runtimes: libc++,
libc++abi and libunwind. We don't need the type to be a string since
there's only binary choice between libgcc and compiler-rt unlike in
the case of C++ library where there're multiple options.

Differential Revision: https://reviews.llvm.org/D49325

llvm-svn: 337116