Add experimental xtoolchain ports for llvm-devel and llvm40.

With an appropriate port installed, in principle you can build a cross
world and/or kernel without a bootstrap compiler or binutils with:

make CROSS_TOOLCHAIN=llvm40 TARGET=arm64 buildworld buildkernel

In practice, this only has a chance of working on platforms with solid
LLD support. At the moment I've had some success with i386, amd64, and
arm64.

Sponsored by: DARPA, AFRL