Switch mesa and related ports to llvm80

Switch mesa over to use llvm80 instead of llvm60. Make it use the
global LLVM_DEFAULT instead of deciding for ourself which llvm version
to use. [1]

Fix build of lang/beginet [1]

Add patch from upstream to fix build of devel/libclc. The patch is taken
from the git mirror of devel/libclc rather than the SVN repo, for
convenience.

Add a patch from mesa upstream preventing certain error messages when using
amdgpu [2]

Add a notice to bsd.default-versions.mk asking that the graphics team be
informed before the llvm version is changed.

Enable llvm and gallium on MIPS. As far as I can tell, this used to
be the default before this change.

Bump portrevisions since dependencies changed.

PR: 230789 [1], [2]
Submitted by: jbeich [1], tobik [2]
Obtained from: FreeBSD Graphics Team development repo

		https://github.com/FreeBSDDesktop/freebsd-ports/commits/feature/mesa-llvm80

Sponsored by: B3 Init (zeising)