Make sure libLLVM users link with libatomic if needed

64-bit atomics are used in llvm/ADT/Statistic.h, which means that users
of libLLVM.so might also have to link with libatomic. To avoid having
to special-case the library here, we simply add all `LLVM_SYSTEM_LIBS`
as public link dependencies to libLLVM.

This fixes a build failure on PowerPC 32-bit.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D132799