rust for mipsel / mips32

[Havard Eidnes <he%NetBSD.org@localhost>]

Hi,

I've finally managed to get rust working on mipsel.  The downside
is that you need a cpu supporting the mips32 instruction set
variant to be able to run it.  As I wrote in the wip/rust commit
message:

    rust: fix our "mipsel" target so that it can work.
    
    Earlier failed attempts:
     * cpu = "mips1", would have been more "portable" to older CPUs,
       but llvm failed to generate code for this cpu in at least one
       instance.  The "mips1" target in llvm is marked "highly experimental",
       so this is perhaps not surprising.
     * cpu = "mips3", an earlier mistake.  Produces "of course" 64-bit
       instructions, so gives "illegal instruction" on e.g. cobalt and
       32-bit-only CPUs.  Despite this also being marked "highly experimental"
       in llvm, this build completed.
    
    So ... we give up some portability to the older CPUs, and generate
    code for cpu = "mips32" with soft-float, and this produces a working
    rustc / cargo combination, at least to the point that devel/cbindgen
    can be built using a MIPSSIM kernel on qemu.
    
    A native build of rust has yet to be attempted, it will be difficult
    with the 508MB max memory limit for the 32-bit mips port, so the
    next best is to try in a o32 chroot on a mips64 kernel which can
    now get more memory.

To aid in testing, I've locally updated the rust-bin package,
installed it on my MIPSSIM host, and done "pkg_tarup" on it.  The
resulting binary package can be fetched from

  ftp://ftp.netbsd.org/pub/NetBSD/misc/he/rust-bin-mipsel-mips32-9.3-1.66.0.tgz
or
  https://cdn.netbsd.org/pub/NetBSD/misc/he/rust-bin-mipsel-mips32-9.3-1.66.0.tgz

You can "pkg_add" this file, and if you have a CPU supporting the
mips32 instruction set and a kernel which does too, you can use
it.

Best regards,

- Håvard