Large integer polyfills (libcore)

[nazar-pc]

I faced a small challenge: my CPU code I'm trying to run on a GPU naturally maps onto u64 and u128, but even u64 apparently requires OpCapability Int64, let alone u128.

Now writing polyfills that work around that feels like a job that compiler should do for me, though I appreciate that it at least tells me that there isn't a native way to run that and I should think about what to do with it.

I'm wondering it it'd be possible to either have a compilation flag that automatically inserts polyfills for such types. It will be an explicit opt-in feature, but otherwise require the least amount of effort for those who do need it. I believe LLVM is capable of this.

I'm really not looking forward to writing polyfills for u128 using a bunch of u32s.