lang/ghc: fix build on powerpc64 elfv1

Dtrace doesn't seem to work on elfv1 powerpc64 (at least on 12.0-RELEASE).

Also properly detect when running on elfv1 and elfv2.

PR: 239193
Approved by: arrowd (maintainer), tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D20995