README.clang: update FreeBSD instructions

The mentioned binutils port got removed while the patch was
pending. As Ian pointed out there is another port providing
the binutils for arm now. Update the instructions accordingly.

Cc: ian@FreeBSD.org
Cc: Tom Rini <trini@ti.com>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>