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

This adds everything that was in some way blocked on const eval, since rust-lang/rust#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: rust-lang/rust#116909