Fix loongarch64 soft-float build (libffi#816) (libffi#817)

Author: scylaac

src/loongarch64/ffi.c

@@ -59,7 +59,9 @@
 */
 typedef struct call_context
 {
+#if !defined(__loongarch_soft_float)
   ABI_FLOAT fa[8];
+#endif
   size_t a[10];
 } call_context;