64-bit atomics are used in llvm/ADT/Statistic.h, which means that users
of libLLVM.so might also have to link with libatomic. Ideally we would
properly distinguish between public and private dependencies on the
component level, but that's of course difficult. Collecting all
dependencies from e.g. Support might also be too much.
This fixes a build failure on PowerPC 32-bit.