Add a .rustfmt.toml with style edition 2024

Use the 2024 style edition for all crates and enable import sorting.
2024 already applies some smaller heuristics that look good in
compiler-builtins, I have dropped `use_small_heuristics` that was set in
`libm` because it seems to negatively affect the readibility of anything
working with numbers (e.g. collapsing multiple small `if` expressions
into a single line).

Author: tgross35

libm/.rustfmt.toml renamed to .rustfmt.toml

@@ -1,5 +1,4 @@
 # This matches rustc
 style_edition = "2024"
-use_small_heuristics = "Max"
 group_imports = "StdExternalCrate"
 imports_granularity = "Module"