src.conf: add WITH_TOOLCHAIN description

It is not used by the in-tree default configuration, but adding it
allows downstream projects with different defaults to make use of
Cirrus-CI (as the makeman test added in 85e8c2a03490 requires that
there are no missing option descriptions).

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39936