[CMake] Enable LLVM_ENABLE_PER_TARGET_RUNTIME_DIR by default on *BSD

Similar to D107799 but for *BSD (DragonFlyBSD, FreeBSD, NetBSD, OpenBSD, etc).
This Linux default has been in main and release/15.x for a while.

`CMAKE_SYSTEM_PROCESSOR MATCHES "^arm"` is excluded for now.

Link: https://discourse.llvm.org/t/rfc-time-to-drop-legacy-runtime-paths/64628

Reviewed By: dim

Differential Revision: https://reviews.llvm.org/D110126