multimedia/obs-studio: fix build on powerpc64?\005

Merge upstream commit to use GCC's SSE->AltiVec translation. Since it depends on compiling with GCC, it only works on ELFv1. Hopefully it will be possible to build it on ELFv2 in the future.

Also use luajit only where it's actually available. Since it's optional, the port builds anyway.

PR: 243199
Approved by: yuri (maintainer)
MFH: 2020Q1 (fix build blanket)