Collection of issues blocked on apfloat situation

[RalfJung]

apfloat is in a bit of a situation and has been for a long time. I'd like to have a central list of things to get back to and fix once that situation is resolved; that is the purpose of this issue.

• apfloat bug: incorrect handling of infinities in FMA #100233
• Sign of zero from differs between CTFE and normal execution (float % float) #102403
• Subnormal f64 to f32 cast is incorrect in CTFE #113407

[eddyb]

I posted a longer update to #55993 (comment) regarding the progress with the whole rustc_apfloat situation, but of note for this issue is that the WIP repo already has 2/3 issues listed here fixed (and the last one has a known fix, just requires further advancing the port closer to the present day).