LLVM backend turns LLVM builtin variables into aliases

The LLVM's aliasification logic (see Note [Llvm Forward References]) currently turns all top-level LLVM variables, including builtins (e.g. $llvm.global_ctors) into aliases. This confuses LLVM, subtly breaking a number of things (e.g. initializers). This causes quite a few testsuite failures in LLVM-only platforms (e.g. ARMv7).