libstdc++: Use glibc >= 2.33 mallinfo2 function

mallinfo started to be deprecated which makes performance tests failed 
to build, just
adopt mallinfo2.

libstdcxx-v3/ChangeLog:

         * testsuite/util/testsuite_performance.h (__mallinfo): New, our 
own mallinfo
         struct with just what we need. When using glibc >= 2.33 use 
mallinfo2 to
         populate it.

Tested under Linux x86_64,

Ok to commit ?

François