protobuf: limit to supported architectures

protobuf 2.5.0 adds an atomic operations implementation that is limited
to a few architectures only. mips64el and armeb don't work either.

Fixes (among others)
http://autobuild.buildroot.net/results/ae9fa612d0b6c9d593504206d5cedd480ad2547e/



Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>