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

[arichardson]

Summary

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.

Test Plan

With this script I've been able to compile on Linux+MacOS (once I've applied
all the other patches from https://github.com/arichardson/cheribsd/tree/crossbuild-aug2018)

[jhb]

Probably best to save this commit for last as once you commit it, people will try it. The change generally works fine for me.

tools/build/make.py

153

We should fix bsd.compiler.mk if it isn't too gross. I recently changed it to handle GCC from base/gcc installed as /usr/bin/cc in rS333414.

[bwidawsk]

I've tested the latest in github today on Arch Linux and was able to build world and kernel - however at this moment, I haven't been able to test what has been built anywhere.

[arichardson]

update to latest version

[arichardson]

Pass -fuse-ld=$XLD when building with clang

[emaste]

I gave this a cursory glance and it seems broadly reasonable. I think it's fine to commit to tools/.

[arichardson]

I'll commit this once all the dependent patches have been committed.

[arichardson]

Stop setting -fuse-ld=, this should be handled by D26055