lang/ghc: Update to 8.4.4 and factor out common port code into bsd.ghc.mk in preparation for adding a port for previous version.

ARM testing by Mikael Urankar <mikael.urankar@gmail.com>.

Submitted by: Viktor Dukhovni <ietf-dane@dukhovni.org> (some patches)
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D17866