package/asterisk: fix build failure due to gcc bug 93847

package/asterisk: fix build failure due to gcc bug 93847

The asterisk package exhibits gcc bug 93847 when built for the Nios2
architecture with optimization enabled, which causes a build failure.

As done for other packages in Buildroot work around this gcc bug by
setting optimization to -O0 if BR2_TOOLCHAIN_HAS_GCC_BUG_93847=y.

Fixes:
http://autobuild.buildroot.net/results/24c0a6ca3b272711a1e6ceaa033925182d0d49c4

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>