Rollup merge of #140323 - tgross35:cfg-unstable-float, r=Urgau

Implement the internal feature `cfg_target_has_reliable_f16_f128`

Support for `f16` and `f128` is varied across targets, backends, and backend versions. Eventually we would like to reach a point where all backends support these approximately equally, but until then we have to work around some of these nuances of support being observable.

Introduce the `cfg_target_has_reliable_f16_f128` internal feature, which provides the following new configuration gates:

* `cfg(target_has_reliable_f16)`
* `cfg(target_has_reliable_f16_math)`
* `cfg(target_has_reliable_f128)`
* `cfg(target_has_reliable_f128_math)`

`reliable_f16` and `reliable_f128` indicate that basic arithmetic for the type works correctly. The `_math` versions indicate that anything relying on `libm` works correctly, since sometimes this hits a separate class of codegen bugs.

These options match configuration set by the build script at [1]. The logic for LLVM support is duplicated as-is from the same script. There are a few possible updates that will come as a follow up.

The config introduced here is not planned to ever become stable, it is only intended to replace the build scripts for `std` tests and `compiler-builtins` that don't have any way to configure based on the codegen backend.

MCP: https://github.com/rust-lang/compiler-team/issues/866
Closes: https://github.com/rust-lang/compiler-team/issues/866

[1]: 555e1d0386/library/std/build.rs (L84-L186)

---

The second commit makes use of this config to replace `cfg_{f16,f128}{,_math}` in `library/`. I omitted providing a `cfg(bootstrap)` configuration to keep things simpler since the next beta branch is in two weeks.

try-job: aarch64-gnu
try-job: i686-msvc-1
try-job: test-various
try-job: x86_64-gnu
try-job: x86_64-msvc-ext2

compiler/rustc_span/src/symbol.rs

@@ -614,6 +614,7 @@ symbols! {
         cfg_target_feature,
         cfg_target_has_atomic,
         cfg_target_has_atomic_equal_alignment,
+        cfg_target_has_reliable_f16_f128,
         cfg_target_thread_local,
         cfg_target_vendor,
         cfg_trace: "<cfg>", // must not be a valid identifier
@@ -2068,6 +2069,10 @@ symbols! {
         target_has_atomic,
         target_has_atomic_equal_alignment,
         target_has_atomic_load_store,
+        target_has_reliable_f128,
+        target_has_reliable_f128_math,
+        target_has_reliable_f16,
+        target_has_reliable_f16_math,
         target_os,
         target_pointer_width,
         target_thread_local,