Support s390

[ojeda]

IBM Z / LinuxONE is supported by Red Hat and Ubuntu, which means we should eventually support the s390x architecture.

Linux s390 targets with official Ubuntu binutils etc. support:

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

s390        s390x-softmmu  SystemZ      binutils-s390x-linux-gnu  s390x-unknown-linux-gnu