Restore functionality of --sysroot on FreeBSD after b18cb9c4

After b18cb9c4, clang would sometimes prefer the host C++ includes
(e.g. in /usr/include/c++/v1) before those specified via --sysroot.
While this behavior may be desirable on Linux, it is not so on FreeBSD,
where we make extensive use of --sysroot during the build of the base
system.  In that case, clang must *not* search outside the sysroot,
except for its own internal headers.

Add an override addLibCxxIncludePaths() to restore the old behavior,
which is to simply append /usr/include/c++/v1 to the specified sysroot.
While here, apply clang-format to the FreeBSD specific toolchain files.

Fixes PR44923.