Disallow loading code on x86_64-unknown-none

[alexcrichton]

... And then also add an escape hatch to allow loading code. This commit is the culmination of discussion on #11506 with a proposed resolution for Wasmtime. The resolution being:

• Wasmtime will reject loading code on x86_64-unknown-none platforms by default.
• A new Config::x86_float_abi_ok escape hatch is added to bypass this check.
• Documentation/errors are updated around x86_float_abi_ok to document the situation.
• The min-platform example is updated to showcase how this is valid to run in that particular embedding (aka enable more features and sufficiently detect said features).

The basic tl;dr; is that we can't detect in stable Rust what float ABI is being used so therefore we pessimistically assume that x86_64-unknown-none is using a soft-float ABI. This is incompatible with libcalls unless they aren't actually called which is only possible when sufficiently many target features are enabled.

The goal of this commit is to be a relatively low-effort way to place a roadblock in the way of "ok ABIs are weird" but at the same time enable getting around the roadblock easily. Additionally the roadblock points to documentation about itself to learn more about what's going on here.

Closes #11506

[alexcrichton]

For posterity @fitzgen I tried to get min-platform working on aarch64-unknown-none but there's relocation errors about compiling libembedding.a (Rust code for aarch64-unknown-none) into a Linux shared library. I don't know how to resolve the errors without first going off and memorizing a bunch of relocation/shared-object things about aarch64...

[cfallin]

Thanks for following up from our Cranelift meeting discussion! A minor documentation thought below in the example but otherwise this looks workable for us at least...

[jsturtevant]

All of these features are required to bypass the libcalls or just a subset?

[alexcrichton]

SE4.1 is sufficient if you're not using relaxed-simd, but with relaxed-simd you'll also need FMA.

[github-actions]

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