Remove brk and sbrk from arm64. They were defined in The Single UNIX
Specification, Version 2, but marked as legacy, and have been removed from
later specifications. After 12 years it is time to remove them from new
architectures when the main use for sbrk is an invalid method to attempt
to find how much memory has been allocated from malloc.

There are a few places in the tree that still call sbrk, however they are
not used on arm64. They will need to be fixed to cross build from arm64,
but these will be fixed in a follow up commit.

Old copies of binutils from ports called into sbrk, however this has been
fixed around 6 weeks ago. It is advised to update binutils on arm64 before
installing a world that includes this change.

Reviewed by: brooks, emaste
Obtained from: brooks
Relnotes: yes
Sponsored by: ABT Systems Ltd
Differential Revision: https://reviews.freebsd.org/D6464