Add math functions for f16 and f128

This adds missing functions for math operations on the new float types.

Platform support is pretty spotty at this point, since even platforms
with generally good support can be missing math functions.
`std/build.rs` is updated to reflect this.

library/std/build.rs

@@ -85,6 +85,11 @@ fn main() {
     println!("cargo:rustc-check-cfg=cfg(reliable_f16)");
     println!("cargo:rustc-check-cfg=cfg(reliable_f128)");
 
+    // This is a step beyond only having the types and basic functions available. Math functions
+    // aren't consistently available or correct.
+    println!("cargo:rustc-check-cfg=cfg(reliable_f16_math)");
+    println!("cargo:rustc-check-cfg=cfg(reliable_f128_math)");
+
     let has_reliable_f16 = match (target_arch.as_str(), target_os.as_str()) {
         // Selection failure until recent LLVM <https://github.com/llvm/llvm-project/issues/93894>
         // FIXME(llvm19): can probably be removed at the version bump
@@ -128,10 +133,42 @@ fn main() {
         _ => false,
     };
 
+    // These are currently empty, but will fill up as some platforms move from completely
+    // unreliable to reliable basics but unreliable math.
+
+    // LLVM is currenlty adding missing routines, <https://github.com/llvm/llvm-project/issues/93566>
+    let has_reliable_f16_math = has_reliable_f16
+        && match (target_arch.as_str(), target_os.as_str()) {
+            // Currently nothing special. Hooray!
+            // This will change as platforms gain better better support for standard ops but math
+            // lags behind.
+            _ => true,
+        };
+
+    let has_reliable_f128_math = has_reliable_f128
+        && match (target_arch.as_str(), target_os.as_str()) {
+            // LLVM lowers `fp128` math to `long double` symbols even on platforms where
+            // `long double` is not IEEE binary128. See
+            // <https://github.com/llvm/llvm-project/issues/44744>.
+            //
+            // This rules out anything that doesn't have `long double` = `binary128`; <= 32 bits
+            // (ld is `f64`), anything other than Linux (Windows and MacOS use ``f64`), and `x86`
+            // (ld is 80-bit extended precision).
+            ("x86_64", _) => false,
+            (_, "linux") if target_pointer_width == 64 => true,
+            _ => false,
+        };
+
     if has_reliable_f16 {
         println!("cargo:rustc-cfg=reliable_f16");
     }
     if has_reliable_f128 {
         println!("cargo:rustc-cfg=reliable_f128");
     }
+    if has_reliable_f16_math {
+        println!("cargo:rustc-cfg=reliable_f16_math");
+    }
+    if has_reliable_f128_math {
+        println!("cargo:rustc-cfg=reliable_f128_math");
+    }
 }