It's a fairly straightforward port of `musl`'s `rintl`, like `rint` and `rintf` were. `libc-test` tests for `rintl` are uncommented since they're now passing.
I've also covered special cases for `rint` with tests, and broke down the current `rint` and `modf` test declarations into multiple -- so each libc function get its own test declaration at the very least.
Contributes to #30978
Reviewed-on: https://codeberg.org/ziglang/zig/pulls/31791
Reviewed-by: Andrew Kelley <andrew@ziglang.org>
The closest namespace the pi/4 constant could belong to is `trig.zig` since
it's used across trig function implementations. On the other hand, chucking
`long double` bit slicing functions into `trig.zig` seems a little more
awkward, so they're put into their own namespace.
Additionally, add helper functions for fetching the sign+exponent
and top 16 bits of a f80/f128's mantissa.
We'll need these functions to implement `cosl`, `sinl`, `tanl`, and,
transitively, `sincosl`.
mihael