I don’t make any test with cross compiling a custom FreeBSD kernel with this toolchain setup under Linux.
But it must be possible. If you’re very lucky, all you need to set LD_LIBRARY_PATH and PATH environment variables to run cross-compiler binaries.
I’m not a BSD guy but probably nobody wants to cross-compile FreeBSD kernel within Linux. I just tried to help people who need to compile their own software project for FreeBSD in Linux. Although cross-compiling FreeBSD kernel itself is theoretically possible, it can be very hard.