MFC r334176:

Indicate the brk/sbrk are deprecated and not portable.

More firmly suggest mmap(2) instead.

Include the history of arm64 and riscv shipping without brk/sbrk.

Mention that sbrk(0) produces unreliable results.

Approved by: re (kib)
Reviewed by: emaste, Marcin Cieślak
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D15535