Enable f128 -> f16 tests on Linux

Since updating the docker images in
<#625>, it looks like
`__extendhftf2` and `__trunctfhf2` are available on all 64-bit Linux
platforms.

testcrate/build.rs

@@ -37,13 +37,10 @@ fn main() {
         features.insert(Feature::NoSysF16F128Convert);
     }
 
-    if target.starts_with("i586") || target.starts_with("i686") {
+    if target.starts_with("i586") {
         // 32-bit x86 seems to not have `__fixunstfti`, but does have everything else
         features.insert(Feature::NoSysF128IntConvert);
-    }
-
-    if target.contains("-unknown-linux-") {
-        // No `__extendhftf2` on x86, no `__trunctfhf2` on aarch64
+        // FIXME: i586 has a bg in `f128 -> f16`
         features.insert(Feature::NoSysF16F128Convert);
     }