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

@@ -14,7 +14,7 @@ struct Target {
 
 impl Target {
     fn from_env() -> Self {
-        let little_endian = match env::var("CARGO_CFG_TARGET_LITTLE_ENDIAN").unwrap().as_str() {
+        let little_endian = match env::var("CARGO_CFG_TARGET_ENDIAN").unwrap().as_str() {
             "little" => true,
             "big" => false,
             x => panic!("unknown endian {x}"),
@@ -31,7 +31,7 @@ impl Target {
                 .parse()
                 .unwrap(),
             little_endian,
-            features: env::var("CARGO_CFG_TARGET_FEATURES")
+            features: env::var("CARGO_CFG_TARGET_FEATURE")
                 .unwrap()
                 .split(",")
                 .map(ToOwned::to_owned)
@@ -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 +260,49 @@ 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 so symbols are provided when `long double` is `f128` which prevents linking
+    // errors on musl (musl makes use of `long double`).
+    //
+    // HACK: this logic should be in `rust-lang/rust` so changes only need to be done in one
+    // place. However, there is no straightforward way to do this, so we do it here.
+    let (f16_ok, f128_ok) = match target.arch.as_str() {
+        // No selection failures
+        "aarch64" | "x86" | "x86_64" | "riscv32" | "riscv64" | "mips" | "mips64" => (true, true),
+        // `f16` selection failure https://github.com/llvm/llvm-project/issues/93894
+        "loongarch" | "loongarch64" => (false, true),
+        // conversion selection failure. `f128` is needed because of `long double`, `f16` is more
+        // optional. <https://github.com/llvm/llvm-project/issues/92866>.
+        "powerpc" | "powerpc64" => (false, true),
+        // No `f16` support <https://github.com/llvm/llvm-project/issues/50374>
+        "s390x" => (false, true),
+        _ => (false, false),
+    };
+
+    // If the feature is set, disable these types.
+    let disable_both = env::var_os("CARGO_FEATURE_NO_F16_F128").is_some();
+
+    println!("cargo:warning=f16 {f16_ok} f128 {f128_ok} disable both {disable_both}");
+
+    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");
+        println!("cargo:warning=f16 enabled");
+    }
+
+    if f128_ok && !disable_both {
+        println!("cargo::rustc-cfg=f128_enabled");
+        println!("cargo:warning=f128 enabled");
+    }
+}
+
 #[cfg(feature = "c")]
 mod c {
     use std::collections::{BTreeMap, HashSet};