[libc++] Don't build against libcxxrt by default on FreeBSD (#84484)

The libc++ CMake build aims to provide a general-purpose configuration
that matches the canonical LLVM setup (libc++ / libc++abi / libunwind),
not the configuration used for compatibility on any given system. For
these "compatibility" configurations, we use CMake caches like
Apple.cmake and FreeBSD.cmake.

Defaulting to libcxxrt on FreeBSD makes it look as though we're trying
to build the system compatible configuration on FreeBSD, which isn't
really correct and causes confusion more than anything else.

Fixes #84476