devel/llvm-{cheri,morello}: Default ABI should be purecap and set various -Xclang

[jrtc27]

It continues to cause confusion that cc and clang (/ clang-{cheri,morello}) do different things. Rather than making cc be the wrapper, we should push that down into the actual clang-{cheri,morello} and have cc just be a symlink.

[kwitaszczyk]

#157 intended to fix this but should be improved to ship only one clang as /usr/local/bin/clang without /usr/bin/clang.