[runtimes] Fix passing lists to runtimes configures

We have to replace the ";" with "|" (since LLVMExternalProjectUtils uses
"|" as the `LIST_SEPARATOR` when invoking `ExternalProject_Add`) in
order for lists to be passed correctly to the runtimes CMake configures.
Remove the special case for `LLVM_ENABLE_RUNTIMES`, since it'll just get
handled by the general logic now.

Differential Revision: https://reviews.llvm.org/D73512