Fix default stack guard.

[Tom de Vries]

Hi,

the following tests fail with glibc trunk for i686-pc-linux-gnu running on an
ubuntu 2.6.24-28-server kernel:
- elf/tst-stackguard1
- elf/tst-stackguard1-static
- nptl/tst-stackguard1
- nptl/tst-stackguard1-static

The tests fail because the default stack guard in the testcases is not the same
as the one in _dl_setup_stack_chk_guard in sysdeps/unix/sysv/linux/dl-osinfo.h.

The reasons for which we choose (in _dl_setup_stack_chk_guard) to use the
default stack guard are:
- __ASSUME_AT_RANDOM is not defined. This is because we compile with
  --enable-kernel=2.6.9. AT_RANDOM is supported from 2.6.29 onwards.
- _dl_random is NULL.  This is because the ubuntu 2.6.24-28-server kernel
  doesn't support AT_RANDOM, so it doesn't pass it in the auxiliary vector, so
  _dl_random is not set in _dl_sysdep_start.
- ENABLE_STACKGUARD_RANDOMIZE is not defined, because
  --enable-stackguard-randomization is not used.

The patch fixes the failing test-cases by restoring the default stack guard to
what it was before the commit 'Make stack canary value harder to read through
read overflow' (15a856b1090669df0aec536edbdf240e71a470ca).

Tested on i686-pc-linux-gnu/2.6.24-28-server system.

Thanks,
- Tom

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