libc: Centralize non-building of sbrk stubs

The actual implementation of sbrk(2) is on top of the undocumented
break(2) system call. On powerpc* this means we don't build _sbrk and
__sys_sbrk which were neither used nor exposed for linkage. Otherwise
it is a no-op.

The addition to lib/libc/sys/Makefile.inc is a direct commit to
stable/14 in lieu of merging the removal of the sbrk and sstk syscalls.

(cherry picked from commit 7893419d492c40ca82b68fca3dcc0f5f7047d39b)

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D43159