graphics/mesa-dri: fix build on powerpc64 elfv2

Base LLVM9 fails to build this port with clang crash (reported on https://bugs.llvm.org/show_bug.cgi?id=43862). The port builds when using LLVM8. Since we depend on LLVM8 anyway, use it also for compilation on elfv2.

Approved by: zeising (maintainer), tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D22274