Configure which platforms get f16 and f128 enabled by default

By moving the logic for which platforms get symbols to
`compiler_builtins` rather than rust-lang/rust, we can control where
symbols get enabled without relying on Cargo features. Using Cargo
features turned out to be a problem in [1].

This will help resolve errors like [2].

[1]: rust-lang/rust#128358
[2]: rust-lang/rust#128401

build.rs

@@ -42,11 +42,12 @@ impl Target {
 
 fn main() {
     println!("cargo:rerun-if-changed=build.rs");
-    configure_check_cfg();
-
     let target = Target::from_env();
     let cwd = env::current_dir().unwrap();
 
+    configure_check_cfg();
+    configure_f16_f128(&target);
+
     println!("cargo:compiler-rt={}", cwd.join("compiler-rt").display());
 
     // Activate libm's unstable features to make full use of Nightly.
@@ -259,6 +260,47 @@ fn configure_check_cfg() {
     println!("cargo::rustc-check-cfg=cfg(assert_no_panic)");
 }
 
+/// Configure whether or not `f16` and `f128` support should be enabled.
+fn configure_f16_f128(target: &Target) {
+    // Set whether or not `f16` and `f128` are supported at a basic level by LLVM. This only means
+    // that the backend will not crash when using these types. This does not mean that the
+    // backend does the right thing, or that the platform doesn't have ABI bugs.
+    //
+    // We do this here rather than in `rust-lang/rust` because configuring via cargo features is
+    // not straightforward.
+    //
+    // Original source of this list:
+    // <https://github.com/rust-lang/compiler-builtins/pull/652#issuecomment-2266151350>
+    let (f16_ok, f128_ok) = match target.arch.as_str() {
+        // `f16` and `f128` both crash <https://github.com/llvm/llvm-project/issues/94434>
+        "arm64ec" => (false, false),
+        // `f16` crashes <https://github.com/llvm/llvm-project/issues/50374>
+        "s390x" => (false, true),
+        // `f128` crashes <https://github.com/llvm/llvm-project/issues/96432>
+        "mips64" | "mips64r6" => (true, false),
+        // `f128` crashes <https://github.com/llvm/llvm-project/issues/101545>
+        "powerpc64" if &target.os == "aix" => (true, false),
+        // `f128` crashes <https://github.com/llvm/llvm-project/issues/41838>
+        "sparc" | "sparcv9" => (true, false),
+        // Most everything else works as of LLVM 19
+        _ => (true, true),
+    };
+
+    // If the feature is set, disable these types.
+    let disable_both = env::var_os("CARGO_FEATURE_NO_F16_F128").is_some();
+
+    println!("cargo::rustc-check-cfg=cfg(f16_enabled)");
+    println!("cargo::rustc-check-cfg=cfg(f128_enabled)");
+
+    if f16_ok && !disable_both {
+        println!("cargo::rustc-cfg=f16_enabled");
+    }
+
+    if f128_ok && !disable_both {
+        println!("cargo::rustc-cfg=f128_enabled");
+    }
+}
+
 #[cfg(feature = "c")]
 mod c {
     use std::collections::{BTreeMap, HashSet};