Update mallinfo2 ABI, and test

This patch adds the ABI-related bits to reflect the new mallinfo2
function, and adds a test case to verify basic functionality.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

malloc/Versions

@@ -64,6 +64,9 @@ libc {
   GLIBC_2.26 {
     reallocarray;
   }
+  GLIBC_2.33 {
+    mallinfo2;
+  }
   GLIBC_PRIVATE {
     # Internal startup hook for libpthread.
     __libc_malloc_pthread_startup;