cilium-builder: Configure clang link to clang-7
cilium-builder: Configure clang link to clang-7

Link clang to clang-7 so that we don't have to litter clang versioning
across various other places in the codebase. When the apt package
version is updated, the developer should also update the alternative on
this newly introduced line.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

cilium/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium-team/cilium,michi-covalent/cilium,cilium-team/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium