Try to fix a periodic windows build failure due to broken rwildcard macro
Update LLVM
Fix build issues once LLVM has been upgraded

* LLVM now has a C interface to LLVMBuildAtomicRMW
* The exception handling support for the JIT seems to have been dropped
* Various interfaces have been added or headers have changed
Fix setting the fixed stack segment attribute on LLVM functions

At the same time create a more robust wrapper to try to prevent this type of
issue from cropping up in the future.
Turn off fast_ffi for some rarely-used LLVM calls
auto merge of #7115 : alexcrichton/rust/llvm-upgrades, r=Aatch

This is a reopening of #6713

This is still blocked on windows failures. I'll re-push try once the existing crisis has passed.