Add support for invoking ld.so via execve() and friends

Introduction:

The original implementation of `relibc_ld_so_start` assumes that
ld.so will always be invoked manually as in "/lib/ld64.so ./a.out"
The problem is regarding this snippet.
    if sp.argc < 2 {
      eprintln!("ld.so [executable] [arguments...]");
      unistd::_exit(1);
      loop {}
    }

As such, In linux when user types "./a.out" he will recieve the message
    ld.so [executable] [arguments...]

This patch makes use of AUXV, specifically AT_ENTRY. When invoking ld.so
manually, AT_ENTRY happens to be the entry point of ld.so. But when
running `./a.out` directly, AT_ENTRY becomes the entry point of `a.out`
this patch compares AT_ENTRY to the entry point of ld.so, if they are
equal only then it will assume that argv[1] is the real program and
adjust the stack, otherwise it will proceed with the stack unadjusted.