Update which platforms have no f16 symbols

Previously we were building the C versions of these symbols. Since we
added the Rust version and updated compiler builtins, these are no
longer available by default. This is unintentional, but it gives a
better indicator of which symbol versions are not actually provided by
the system.

Use the list of build failures to correct the list of platforms that do
not have `f16` symbols.

Author: tgross35

testcrate/build.rs

@@ -44,10 +44,20 @@ fn main() {
         features.insert(Feature::NoSysF16F128Convert);
     }
 
-    if target.starts_with("wasm32-") {
+    // These platforms do not have f16 symbols available in their system libraries, so
+    // skip related tests. Most of these are missing `f16 <-> f32` conversion routines.
+    if (target.starts_with("aarch64-") && target.contains("linux"))
+        || target.starts_with("arm")
+        || target.starts_with("powerpc-")
+        || target.starts_with("powerpc64-")
+        || target.starts_with("powerpc64le-")
+        || target.contains("windows-")
         // Linking says "error: function signature mismatch: __extendhfsf2" and seems to
         // think the signature is either `(i32) -> f32` or `(f32) -> f32`
+        || target.starts_with("wasm32-")
+    {
         features.insert(Feature::NoSysF16);
+        features.insert(Feature::NoSysF16F128Convert);
     }
 
     for feature in features {