Propose to Use madvise API on Runtime Loader

[lin zuojian <manjian2006 at gmail dot com>]

Hi,
    I am trying to reduce the memory foot print of shared objects.After
    examine the layouts of many shared objects, I find out that the start
    part of a shared object will be seldom needed.
    So I add the following code to the runtime loader:
    madvice(si->base, si->rel + si->rel_count * 8, MADV_DONTNEED);

    And Find out the foot print drops proportionally dramatically.
    An very big shared object(libwebcore.so), drops from 7MB to 4MB.
    So I think it's a good idea to add this technique to the future
    eglibc.

    madvice tell *nix system to let go the page table and the page in
    the specify range. But not the vma.
--
Lin Zuojian