Choose a better SIMD-less target than i586 Linux

[clarfonthey]

This has been a slow burn but Linux is slowly removing support for these targets and it's unclear how long they'll be supported: https://www.phoronix.com/news/AMD-K5-CPUs

We should probably choose a different target for testing the generic SIMD code.

[cuviper]

Does that change matter to us? We're not testing on a 32-bit kernel. To a 64-bit kernel, it's just a 32-bit compat process, regardless of whether it abstains from certain CPU features.

[clarfonthey]

Yeah, it's fair to say we're just running i686 minus SIMD here, but I figure regardless it's probably worth considering a target that's more likely to be in active use.

[xtqqczze]

i586-unknown-linux-gnu is still Tier 2 for Rust toolchain.