math/blis: update to 0.4.0

This release incorporates the changes needed for us which were previously only
available through a snapshot. It also allows us to drop some install hacks and
brings general improvements to blis itself.

Switch to llvm6 as the compiler.

Reviewed by: zeising (mentor)
Approved by: zeising (mentor)
Differential Revision: https://reviews.freebsd.org/D16522