Auto merge of #126608 - tgross35:f16-f128-library, r=Mark-Simulacrum

Add more constants, functions, and tests for `f16` and `f128`

This adds everything that was in some way blocked on const eval, since https://github.com/rust-lang/rust/pull/126429 landed. There is a lot of `cfg(bootstrap)` since that is a fairly recent change.

`f128` tests are disabled on everything except x86_64 and Linux aarch64, which are two platforms I know have "good" support for these types - meaning basic math symbols are available and LLVM doesn't hit selection crashes. `f16` tests are enabled on almost everything except for known LLVM crashes. Doctests are only enabled on x86_64.

Tracking issue: https://github.com/rust-lang/rust/issues/116909