onnxruntime with ROCm

[sjnewbury]

Hi! Since you're maintaining this, I thought I'd fix it up to work with the current ROCm in the Gentoo tree.

I did need to create an ebuild for migraphx which took a day to make work! I have onnxruntime building now (for gfx900), but without USE=javascript since there seems to be a problem there. Once I've figured out the javascript isssue, I'll send a PR with migraphx and the changes to the onnxruntime ebuild.

I also had to patch eigen to apply the NaN propagation fix, so I'll probably include an eigen ebuild with the needed patches.