[gn build] Set HAVE_MALLINFO2=1

I'm seeing deprecated warnings due to using mallinfo() instead of
mallinfo2().

../../llvm/lib/Support/Unix/Process.inc:98:10: warning: 'mallinfo' is deprecated [-Wdeprecated-declarations]
  mi = ::mallinfo();

mallinfo2() is part of glibc 2.33 which was released in Feb 2021, which
is fairly recent but I think gn users should be using fairly up to date
glibcs.

If this breaks people we could make this a gn arg instead.

Differential Revision: https://reviews.llvm.org/D117916