Weird x86 feature detection considerations

[valadaptive]

There was some previous discussion about doing feature detection via cpuid on x86, even in no_std builds. There might be some complications--nothing concrete here, but things I stumbled across and want to note here so I don't forger about them:

• The CPUID instruction is not always available, and might segfault
• Weird stuff relating to AVX2 on x86_64-unknown-none
• XGETBV, which you're supposed to use to detect AVX2, is nondeterministic?