math/blis: pacify portlint, add test target, optimize for power9 on powerpc64

1. Move USES block to pacify portlint.
2. Add test target.
3. Add perl as a build dependency, I'm not sure how it worked before.
4. Optimize for power9 on powerpc64. This will break blis on all earlier POWER generations, but nothing depends on this port so I guess it's ok. make test passes fine on both elfv1 and elfv2.
5. Remove LIBNAME, it's not necessary.

PR: 246194
Approved by: jmd (maintainer timeout)