Updates to mesa and llvm

[Philip Balister <philip@balister.org>]

Recently I need to get the llvmpipe driver running with mesa. I ended up
adding a llvm-3.7 recipe and updating mesa to 10.6.8. Not sure if going
to 11.x.y also works.

I'd like to upstream these since they configuration actually worked and
performance was much better. I recall some discussion lately about this
process and wanted to make sure I didn't break anything :)

I'll send on the llvm 3.7 stuff this week, since it should work in
parallel with the older recipe. I do need to move the license file
checksum into the version specific piece.

The timing on the mesa update is trickier since we are trying to
stabilize the oe-core release.

Comments? Will adding another llvm recipe cause trouble for other packages?

Philip