Cool new FreeBSD 11/12 kernel developer trick

One of the cool new features in FreeBSD 11 is that we can use the system compiler as a cross compiler, eliminating the need to rebuild clang. While this allows buildworld to go much faster, there's another benefit that might not be immediately obvious.

For kernel development, this almost means we can forget about the usually required 'make kernel-toolchain' even when we're cross building. The system linker can't (yet) handle it, so the kernel fails to link if you've not done it. The good news is that you don't need to build gcc or clang for the kernel toolchain target, so you can build it quickly (in under a minute on my fast build machine, under 5 on my laptop VM). The trick is one command:
make kernel-toolchain TARGET=arm WITHOUT_{GCC,CLANG}{,_BOOTSTRAP}=t
If you are developing for amv6/v7, change TARGET=arm to TARGET_ARCH=armv6. I've not tried others, but suspect they will work as well as clang supports the target in question. This allows me to start building test kernels w/o the usual delay of building a kernel toolchain, especially on my laptop VM where  I'm chronically short on disk and always deleting the old obj tree.

This works on both 11.0 and 12.0-current. It likely will fail on 10.x since the system compiler as the cross compiler work hasn't been MFC'd there yet.

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