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/malloc.c

@@ -639,6 +639,7 @@ libc_hidden_proto (__libc_mallopt)
   thus be inaccurate.
 */
 struct mallinfo2 __libc_mallinfo2(void);
+libc_hidden_proto (__libc_mallinfo2)
 
 struct mallinfo __libc_mallinfo(void);
 
@@ -4999,6 +5000,7 @@ __libc_mallinfo2 (void)
 
   return m;
 }
+libc_hidden_def (__libc_mallinfo2)
 
 struct mallinfo
 __libc_mallinfo (void)