Mk/Uses/sbrk.mk: centralize sbrk handling

Add a USES=sbrk that sets BROKEN_aarch64 and BROKEN_risc64.  This paves
the way for further deprecation of sbrk and will be extended to support
patching around various uses.

Approved by:	portmgr (mat)
Differential Revision:	https://reviews.freebsd.org/D47258