Remove float32/simd backend extension checks

[TheNumbat]

We currently allow no-extension targets to depend on libraries that use extensions.
The compiler backend also asserts that the float32/SIMD extensions are enabled whenever it encounters a f32/v128 register.

After adding the float32 API to stdlib_stable, this has become a bigger problem.
When linking a no-extension target, the compiler can cross-module inline code from stdlib_stable, so the backend will find f32/v128 temps and assert.
This PR removes the assertions.

It's not clear whether this is the right solution - it might be possible to conditionally disable the checks for imported code, but that seems much harder.
At this point the f32/v128 code paths are pretty well tested, and I don't think we need to worry about catching bad types produced by the middle-end here, so I think it's ok to remove the checks. The regalloc validator can catch allocation issues in the cfg pipelines.

[goldfirere]

I haven’t reviewed the implementation of the change Max writes about, but I think this is fine. Having no-extension code depend on extension code is perhaps strange, but we have other mechanisms in place to stop us from doing this in a bad way; there’s no reason the compiler has to enforce this.