Improve Linux flash0 path handling, fix __cpuidex() in some gccs

This way you don't have to copy anything, it should just work normally as on e.g. Windows.  One less gotcha.

@skardach: does this __cpuidex() work for you as well?  I was trying to test 32-bit (for some jit stuff), and gcc was telling me it had "impossible constraints."  I think this version should work fine, and it compiled for me.  Unfortunately I couldn't get Ubuntu to install the proper 32-bit libs side by side.

-[Unknown]