Change patching of x86 closures/constants

This commit fixes a bug in the LLVM x86 backend that triggers a segfault on
some tests.
Add support for llvm unique symbols in hipe_gensym

This is used by the LLVM backend in order to generate unique names for
temporary SSA values and functions.
Extend RTL API to support the LLVM backend

* Extend the RTL 'call' instruction with a normalcontinuation field to host...
* Extend the 'rtl_var' definition to host liveness information needed for a
simple liveness analysis performed by the LLVM backend.
* Uncomment some function definitions (mostly simple accessors) that are
already there and are useful for the translation of RTL code to LLVM assembly.
Prepare for the LLVM backend

Add flags to enable and use the LLVM backend:
  * to_llvm: use the LLVM pipeline for compilation (default optimization level
  is O3),
  * llvm_save_temps: save the intermediate files in current directory in order
  to be able to debug or optimize the LLVM assembly,
  * {to_llvm, O}: set the optimization level of LLVM opt and llc tools.

Note: This commit doesn't compile because it makes some calls to yet undefined
functions.
Add the LLVM backend in HiPE

HiPE can now use the LLVM pipeline for generating executable code. Some debug
support has also been added to the loader; no semantic change intented.