Rollup merge of #142340 - RalfJung:miri-apfloat-mul-add, r=oli-obk

miri: we can use apfloat's mul_add now

With rust-lang/rustc_apfloat#11 fixed, there is no reason to still use host floats here.
Fixes rust-lang/miri#2995

We already have a test for this:
https://github.com/rust-lang/rust/blob/a7153db254acc387e271e75153bdbd3caa2bed89/src/tools/miri/tests/pass/float.rs#L998-L1003

r? ``@oli-obk``

Author: matthiaskrgr