Use Kernel mapped binaries when available.

At least in linux kernel, assuming that a.out is an elf that is linked
against relibc's own ld.so. When a user attempts `./a.out`, Linux kernel
will map `./a.out`, then map `ld.so` and jump into ld.so entry point.
In relibc ld.so will simply ignore the kernel mapped a.out and create
its own mapping. This patch forces relic ld.so to use the already mapped
`a.out` when ever possible. This would normally have slight performance
improvement (especially that currently relibc doesn't map a.out but
instead copy the data into empty mmaped memory).

The real motivation behind this patch is while impelemnting Runtime
linker debugging protocol for relibc. part of the protocol is ld.so
inseting address of some ld.so managed data structure into .dynamic
seciton of a.out then the debugger would check it there. The thing is
that debuggers have information about the kernel loaded ./a.out and they
check that one specifically which is in our case totally ignored by
relibc.