Add a tools/build/make.py script that bootstraps bmake and then runs the build

This makes it possible to compile on non-FreeBSD systems since make will
usually be GNU make there. Even if they include bmake, it will often
either be a broken version or too old to build FreeBSD.

This should be the last commit needed to compile FreeBSD on Linux+macOS.
After over two years, I've finally managed to upstream all our local CheriBSD
changes to allow building on Linux (and as a result of being reviewed by more
people they are slightly less ugly than they were before).

It should now be possible to run the following to build on Linux+macOS if you
have LLVM/Clang 10 or newer installed:
MAKEOBJDIRPREFIX=/somewhere ./tools/build/make.py TARGET=amd64 TARGET_ARCH=amd64 buildworld

I have only tested macOS 15, Ubuntu 18.04 and openSUSE Leap, but other Linux
distributions might also work (as long as they ship a recent GLibc and compiler).

Reviewed By: emaste (should be fine to commit to tools/)
Differential Revision: https://reviews.freebsd.org/D16767