Shuffle around stdsimd::arch::detect bits

[alexcrichton]

Compile more code on more platforms, tweak imports, try to catch mistakes
sooner.

[alexcrichton]

This is fixing this error which wasn't caught during x86/x86_64 builds. I'm hoping to build a maximal amount of code on all platforms to check as much as we can statically and then most of it just ends up being dead code that's eliminated.

[gnzlbg]

It would be nice to triple-check that the cache is indeed eliminated on platforms that don't need it.

[gnzlbg]

Also in which platforms is this code failing, and why isn't it failing in stdsimd's CI ?

[alexcrichton]

Oh yeah this is for sure all jettisoned from libstd, and it's an issue with integrating into libstd rather than stdsimd itself. In libstd there's no top-level _std item, but rather more tweaks need to happen for reexported items in the stdsimd crate's ci to integrate well into libstd

[gnzlbg]

but rather more tweaks need to happen for reexported items in the stdsimd crate's ci to integrate well into libstd

Ah crap, maybe I screwed it up last time I touched it. Could you add a comment to the top-level crates explaining a bit how things must be done to not break anything in libstd ?

[alexcrichton]

Oh no worries, I've renamed the import of std to __do_not_use_this_import to discourage further use :)