Makefile: add a small blurb about building with gcc xtoolchain

The key details are to install the appropriate flavor of devel/freebsd-gcc6
and pass CROSS_TOOLCHAIN while building.

(cherry picked from commit 5f2aaba4532c713f74279f0e83208c97af3a3e69)
(cherry picked from commit cf82304d7d5e8d9433d46cbdf2db8c2576b85edd)