malloc: Decorate malloc maps

Add anonymous mmap annotations on loader malloc, malloc when it
allocates memory with mmap, and on malloc arena.  The /proc/self/maps
will now print:

   [anon: glibc: malloc arena]
   [anon: glibc: malloc]
   [anon: glibc: loader malloc]

On arena allocation, glibc annotates only the read/write mapping.

Checked on x86_64-linux-gnu and aarch64-linux-gnu.
Reviewed-by: DJ Delorie <dj@redhat.com>