MFC r292268, r334176

r292268:
Remove sys/types.h due to STANDARDS and unistd.h also includes
sys/types.h.

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.

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