Fix AtomicCmpXchg for LLVM 3.5

A weak variant was added to LLVM 3.5 and the return type changed to { iN, i1 }.
Just extract the value from this struct.
Merge branch 'release-0.13.0'
Add more cpu architectures for LLVM 3.5

ppc64le, amd64, amd64_eb and armeb have been added to LLVM 3.5.