Add test using a compiler builtin

This uses the `__floatditf` function which was reported as an issue in
[wasi-libc#543]. Compiling and running this test should prove that the
builtins are available in the `wasm32-*-threads` targets as long as the
tests use those targets (they do, right?).

[wasi-libc#543]: WebAssembly/wasi-libc#543

tests/general/builtin-floatditf.c

@@ -0,0 +1,10 @@
+// Primarily check that `libclang_rt.builtins-wasm32.a` functions are present
+// and work as expected on all tested targets, not the builtin functionality.
+
+#include <assert.h>
+
+int main() {
+  tf_float f = __floatditf(0);
+  assert(f == 0.0);
+  return 0;
+}