cross-tools: Document a rather bizarre looking construct

On Linux, stddef.h will define size_t to something when size_t is
defined. On FreeBSD, size_t is an actual typedef which we need for
other parts of the system. If size_t isn't defined, that tells us we
need to define the fallback size_t typedef (if it is defined, then we
can't easily redefine it without issues because glob.h will define it to
something different than stddef.h defines it). Add a comment to this
effect.

Sponsored by: Netflix
Reviewed by: val_packett.cool
Differential Revision: https://reviews.freebsd.org/D45350