Adding support for overriding LLVM_ENABLE_RUNTIMES for runtimes builds.

Second attempt: Now with ';' -> '|' replacement.

On some platforms, certain runtimes are not supported. For runtimes builds of
those platforms it would be nice if we could disable certain runtimes (ie
libunwind on Windows).

Differential Revision: https://reviews.llvm.org/D67195

llvm-svn: 372784