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

Tracking Issue (target features): rust-lang/rust#150259.
Tracking Issue (-Zpacked-stack): rust-lang/rust#156724.
Tracking Issue (-Ctarget-feature=+backchain): rust-lang/rust#158014.
Lore: https://lore.kernel.org/rust-for-linux/20260608181451.3734956-1-japo@linux.ibm.com/.

[ojeda]

https://lore.kernel.org/rust-for-linux/20260512105920.2426293-1-japo@linux.ibm.com/