Disable sbrk() use in GNU tools.

We're studing the possibility of deprecating sbrk(). To make it easier
we're removing unnecessicary uses in the base system. None of these
tools require sbrk(), but they agressively prefer it for no good reason.

Reviewed by: andrew
Approved by: re (kib)
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D16141