Auto merge of #123351 - beetrees:x86-ret-snan-rust, r=nikic,workingjubilee

Ensure floats are returned losslessly by the Rust ABI on 32-bit x86

Solves #115567 for the (default) `"Rust"` ABI. When compiling for 32-bit x86, this PR changes the `"Rust"` ABI to return floats indirectly instead of in x87 registers (with the exception of single `f32`s, which this PR returns in general purpose registers as they are small enough to fit in one). No change is made to the `"C"` ABI as that ABI requires x87 register usage and therefore will need a different solution.

Author: bors