By having ErLLVM explicitly tell LLVM which architecture we're expecting
it to compile for we remove the risk of having LLVM generate amd64 code
for a x86 VM.

and correct the name of another, erroneously spelt, option in the process.

Named types, except those for the records in the header files, have been
moved from elf*_format.hrl to elf_format.erl.

Also:
 * Use lists instead of element/2-indexed tuples for ELF section and
   symbol tables.
 * Enable +warn_export_vars for hipe/llvm/ subdirectory and fix warning.
 * Extract duplicated code in hipe_llvm module into help function
   pp_dereference_type/3.

This allows for much more robust interpretation of relocations, symbols
and sections in hipe_llvm_main, without the clunkiness of an abstract
interface between two internal modules that belong to the same subsystem
anyway.