Support optionally no_std dependents

[gkgoat1]

I'm making a no_std library that will use iced-x86. It will also allow usage with std, but iced-x86 currently requires setting exactly one of std and no_std, preventing me from creating no_std libraries using it that also support std.

A simple and compatible fix would be to allow (maybe with a warning) setting both std and no_std, with std taking priority.