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.

Reviewed by: emaste, Marcin Cieślak
MFC after: 3 days
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D15535