Skip the f16-related tests on LoongArch

Skip the f16-related tests on LoongArch if there are no f16 symbols
in the system libraries.

Author: heiher

testcrate/build.rs

@@ -75,6 +75,7 @@ fn main() {
         || target.arch == "powerpc"
         || target.arch == "powerpc64"
         || target.arch == "powerpc64le"
+        || target.arch == "loongarch64"
         || (target.arch == "x86" && !target.has_feature("sse"))
         || target.os == "windows"
         // Linking says "error: function signature mismatch: __extendhfsf2" and seems to