lang/sbcl: Hook into the FreeBSD ports asdf framework by depending on
devel/cl-asdf and loading etc/asdf-init.lisp in sbclrc

Also:

• Download SBCL bootstrap binaries from official upstream source instead of the FreeBSD cluster
• Use DISTVERION instead of PORTVERSION
• Fix portlint warnings about ordering
• Use variables; e.g., ${PORTVERSION} instead of repeating sbcl
• Bbump PORTREVISION

Approved by: krion (maintainer)
Differential Revision: https://reviews.freebsd.org/D12055