Build LLVM deps for x86_64 FreeBSD, OpenBSD, and NetBSD

[rtfeldman]

Adds x86_64-freebsd-none, x86_64-openbsd-none, and x86_64-netbsd-none to the release matrix so Roc can be built for the three major BSDs without a system LLVM. Today a BSD user has to point zig build at a ports LLVM via -Dllvm-path, and there is no way to cross-build a BSD-targeted roc from CI at all, because no BSD-targeted LLVM/LLD static libraries exist to link against.

Everything these targets need is already here: build maps freebsd/openbsd/netbsd to their CMake system names, and the repo already carries the OpenBSD llvm-config patch plus the lld findMajMinShlib() patch from OpenBSD ports, which is what lets lld resolve OpenBSD's versioned libc.so.<maj>.<min>. Each target cross-compiles from ubuntu-latest with zig cc, so no BSD runners are involved.

Draft until CI proves the three builds succeed. LLVM's CMake configure runs try_compile probes that link real executables, and that is the step most likely to need iteration on these targets. Once a release is tagged, the roc repo needs a follow-up adding the three tarballs to build.zig.zon and their triples to supported_deps_triples in build.zig.