make.conf.5: add CPUTYPE?= note from UPDATING

Cross-compilation needs CPUTYPE to be set with ?= to be overideable via
make targets. This has historically been mentioned in the FOOTNOTES of
UPDATING. Note that in the CPUTYPE documentation in make.conf(5) for
discoverability. Leave it in UPDATING for now to create no regression in
the doc there.

MFC after: 3 days
Reviewed by: carlavilla, ivy
Approved by: carlavilla (mentor)
Differential Revision: https://reviews.freebsd.org/D50277

(cherry picked from commit bbc0b39ef7df3007c78eec8822213fe77aae4d41)