Makefile: re-wordsmith the blurb about xtoolchain ports

The new version only includes a specific version once, and uses the one
that's currently advised by tinderbox: -gcc6.

It also advises just installing the pkg, but mentions in a side-note at the
end where to find the source in the ports tree.

Reviewed by: jrtc27
Suggested by: jhb (use default from tinderbox)
Differential Revision: https://reviews.freebsd.org/D26820