Add `s390x-unknown-none-softfloat` with `RustcAbi::Softfloat`
followup on rust-lang/rust#150766
add an `s390x-unknown-none-softfloat` target to use for kernel compilation, as the Linux kernel does not wish to pay the overhead of saving float registers by default on kernel switch. this target's `extern "C"` ABI is unspecified, so it is unstable and subject to change between versions, just like the Linux intrakernel ABI and `extern "Rust"` ABIs are unstable.
enforce target feature incompatibility by adding `RustcAbi::Softfloat`. this is itself just a rename of `RustcAbi::X86Softfloat`, accepting both "x86-softfloat" and "softfloat" as valid strings in the target.json format. the target-features of `"soft-float"` and `"vector"` are incompatible for s390x, so issue a compatibility warning if they are combined.
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