mongodb: disable support for armv4 and armv5

When compiling for armv4 or armv5, we get the following error:

src/mongo/util/signal_handlers_synchronous.cpp:188:9: error: 'current_exception' is not a member of 'std'
     if (std::current_exception()) {
         ^

This is because libstdc++ will only enable std::current_exception if ATOMIC_INT_LOCK_FREE > 1, which is not the case for armv4 and armv5 architectures, so disable them.

Fixes:
http://autobuild.buildroot.net/results/589a02f511939531141135def73785b600113152/
http://autobuild.buildroot.net/results/89b9cd2dae8645b50407409c7f2a8567c551822c/
http://autobuild.buildroot.net/results/b8ffce927cb0563f5c43533ec2f02d3487acc198/



Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>