Most of our Rust-specific overflowing intrinsics currently return
`(i128, bool)`, which is not guaranteed to have a stable ABI. Switch to
returning the overflow via a mutable parameter and only directly
returning the integer result.
`__rust_i128_mulo` now matches the function signature of `__muloti4`,
but they do not share the same ABI on Windows so we cannot easily
deduplicate them.
Currently, tests of the same kind are grouped together across all types
into a single function. This makes it difficult to understand exactly
what failed in CI.
Change test macros to create separate functions for separate types so
failures are more fine grained.
Change float test macros to fall back to testing against `rustc_apfloat`
when system implementations are not available, rather than just skipping
tests.
This allows for easier debugging where operations may not be supported.
