Implement Display, LowerExp, and UpperExp, and FromStr for f128.

[bjoernager]

(Please disregard this PR for the moment as it will require a bit more attention).

Tracking issue: #116909.

Of the formatting traits, f128 currently only implements Debug. f16, meanwhile, additionally implements Display, LowerExp, and UpperExp, with a conditional fallback to Debug on some targets.

This PR ports these implementations to f128 as well, with equivalent fallbacks for targets without proper support.

This PR also implements FromStr for f128. On the targets that natively support such numbers, this implementation will work as expected. On other targets, a run-time panic will encur for now (this is similarly done for f16 already).

[rustbot]

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[bjoernager]

@rustbot label +F-f16_and_f128 +T-libs-api -T-libs

r? libs-api

[rust-log-analyzer]

The job aarch64-gnu-llvm-19-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] build_script_build test:false 0.131
[RUSTC-TIMING] cc test:false 0.765
   Compiling compiler_builtins v0.1.160 (/checkout/library/compiler-builtins/compiler-builtins)
[RUSTC-TIMING] build_script_build test:false 0.345
error[E0277]: the trait bound `f128: RawFloat` is not satisfied
  --> library/core/src/num/flt2dec/decoder.rs:68:25
   |
68 | impl DecodableFloat for f128 {
   |                         ^^^^ the trait `RawFloat` is not implemented for `f128`
   |
   = help: the following other types implement trait `RawFloat`:
             f16
             f32
             f64
note: required by a bound in `DecodableFloat`
  --> library/core/src/num/flt2dec/decoder.rs:43:27
   |
43 | pub trait DecodableFloat: RawFloat + Copy {
   |                           ^^^^^^^^ required by this bound in `DecodableFloat`

For more information about this error, try `rustc --explain E0277`.
[RUSTC-TIMING] core test:false 24.346
error: could not compile `core` (lib) due to 1 previous error
Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`

[rust-bors]

☔ The latest upstream changes (presumably #151716) made this pull request unmergeable. Please resolve the merge conflicts.