Use __executable_start as the lowest address for profiling [BZ #28153]

Glibc assumes that ENTRY_POINT is the lowest address for which we need
to keep profiling records and BFD linker uses a linker script to place
the input sections.

Starting from GCC 4.6, the main function is placed in .text.startup
section and starting from binutils 2.22, BFD linker with

commit add44f8d5c5c05e08b11e033127a744d61c26aee
Author: Alan Modra <amodra@gmail.com>
Date:   Thu Nov 25 03:03:02 2010 +0000

            * scripttempl/elf.sc: Group .text.exit, text.startup and .text.hot
            sections.

places .text.startup section before .text section, which leave the main
function out of profiling records.

Starting from binutils 2.15, linker provides __executable_start to mark
the lowest address of the executable.  Use __executable_start as the
lowest address to keep the main function in profiling records. This fixes
[BZ #28153].

Tested on Linux/x86-64, Linux/x32 and Linux/i686 as well as with
build-many-glibcs.py.