build(7): clarify buildenv target can be used for non-cross builds

make buildenv can be used for building for the same architecture as
the host (perhaps this is a degenerate case of cross-building).
TARGET and TARGET_ARCH do not need to be set in this case.

Reviewed by: bdrewery
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D10759