Revert "Revert "[CMake] Enable LLVM_ENABLE_PER_TARGET_RUNTIME_DIR by default on Linux""

This reverts commit 90c9d41c.

Keeping arm* LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=on is sufficient to work around
LLVM_ENABLE_RUNTIMES not working well with arm. It is more important for
LLVM_ENABLE_PROJECTS and LLVM_ENABLE_RUNTIMES to match in the file hierarchy.