Update to 13.0.6

• Allow use of newer LLVM, i.e. llvm40, via MESA_LLVM_VER in make.conf [1]
• Disable use of LLVM on platforms where it's known not to be available [2]

PR: [1] 217016, [2] 216944
Reported by: [1] greenreaper@hotmail.com, [2] linimon
Approved by: swills (mentor)
Differential Revision: https://reviews.freebsd.org/D10183