Add two FreeBSD customizations to libc++

This is intended to better support ports usage of older clang versions:

• Add _LIBCPP_ENABLE_COMPILER_VERSION_CHECKS block around compiler version checks, to avoid any warnings about support. This makes some ports that use -Werror fall over.
• When using clang < 15.0, avoid using a type visibility attribute on the std namespace, as older versions of clang do not support this.

PR: 276104
MFC after: 1 month

(cherry picked from commit 412fa3436f0d1fe4a7e5e3b66783aa40f599125e)