LLVM backend: Remove unused forward reference list

LLVM IR allows forward references cleanly so the backend didn't use the
forward list anymore. This old code came from the C backend and even was
buggy, see caadfc20.
LLVM backend: Add support for global ctors/dtors
LLVM backend: Only declare procedures that don't have a body
C/LLVM backends: Share code for finding global procs/vars to emit
C/LLVM backends: Extract common code for emitting DATAs
LLVM backend: Remove unnecessary init done by _emitBegin() anyways
LLVM backend: Emit fixed-size types just like the C backend

see also 9c7fc49b.