[runtimes] Remove override of LLVM_ENABLE_PER_TARGET_RUNTIME_DIR on Darwin (#213748)

It should be possible to produce a per-target include directory even on
Apple platforms. That's not the way we ship the library by default, but
there's no reason not to allow selecting that configuration.

GitOrigin-RevId: b5c9e1b7bd50ef79fdfa91a7f2292f30889a2fa3