lang/mit-scheme: add BUILD_DEPENDS as:devel/binutils

/usr/bin/as will soon be removed from the FreeBSD base system, for
FreeBSD 13.0.

Depend on as (not ${LOCALBASE}/bin/as) so that this will introduce no
change for existing 11.x and 12.x, which will continue to use the base
system GNU as 2.17.50.

PR: 205250
Approved by: portmgr (blanket)
Sponsored by: The FreeBSD Foundation