Support sparc

[ojeda]

While all Linux, QEMU, LLVM and Rust support SPARC to some degree, there is no official support from any major distribution (Ubuntu, Red Hat or even Debian). Furthermore, while Oracle and Fujitsu still sell servers, they seem to only support Solaris. Furthermore, there is no support for Clang/LLVM-built kernels. Therefore, this is an architecture we can probably skip. Nevertheless, if someone wants to add support, feel free to give it a go!

Linux sparc targets without official Ubuntu binutils etc. support:

Linux arch  QEMU target      LLVM target  Ubuntu binutils  Example Rust triple
----------  ---------------  -----------  ---------------  -------------------------

sparc       sparc-softmmu    Sparc        -                sparc-unknown-linux-gnu
sparc       sparc64-softmmu  Sparc        -                sparc64-unknown-linux-gnu

For 64-bit, one may use binutils-sparc64-linux-gnu from Ubuntu universe or Debian.