musl: no SSP on i386 and PowerPC

musl: no SSP on i386 and PowerPC

Due to what appears a bug in gcc according to the musl developers, but
that the gcc developers don't want to fix, SSP support currently don't
work on i386 and PowerPC with musl.

Additional details can be found at:

  http://www.openwall.com/lists/musl/2016/12/04/2

OpenWRT and Alpine Linux both have musl and gcc patches to work around
the issue, but in the context of Buildroot, we at this point don't care
enough about SSP support specifically with musl on those architectures
to carry additional patches.

Currently, having SSP enabled with musl/i386 causes a number of build
failures in the autobuilders: cups, ipmiutil, openssh, ruby, stunnel,
sudo and mosh at least all fail to build because of this.

So we simply disable SSP support in the toolchain when musl is used on
i386 and PowerPC. The PowerPC case is not tested in the autobuilders,
but has been reproduced locally and is also fixed by this patch.

Fixes:

  mosh
  http://autobuild.buildroot.net/results/60aa12f1aed08e3b7a98f9ce7091bee3a44d692c/

  ipmi-util
  http://autobuild.buildroot.net/results/fb9a071b8739527f424cfe2886ec480f438f70ab/

  cups
  http://autobuild.buildroot.net/results/486dea944d6ecba5c4e6e8ac664261c1909f4b4c/

  openssh
  http://autobuild.buildroot.net/results/742a8bf4726de6e9ba6926e3fb6019a434454e48/

  sudo
  http://autobuild.buildroot.net/results/682531f368c4e982cafe9e625dd41f6d8c7f93f9/

  ruby
  http://autobuild.buildroot.net/results/dac660f96c7f85e933a6b82cf61edd429eeae9aa/

  stunnel
  http://autobuild.buildroot.net/results/cee52505f1ac2da2f5ba86c9ebfd1f5cd9e301be/

Thanks to Yann E. Morin for suggesting to simply disable SSP support
rather than trying to fix it.

[Peter: add comment explaining why]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>