Built a bootkit for NetBSD/aarch64

Also the package is now more explicit about LLVM. It used to invoke whatever llc(1) and opt(1) found in the $PATH when -fllvm is passed to the compiler, though it's not the default backend. Now LLVM is an explicit, optional dependency on platforms where NCG backend is available, and a mandatory dependency on others.

Note that the LLVM backend tends to produce faster code but it runs significantly slower than NCG. AArch64 support of NCG is being worked on, but it hasn't been merged to the 9.0 branch yet (or even the master branch).

(pho)