New support for variable argument functions & intrinsic functions

Two new features are now supported by LLVM:
  1. Intrinsic functions. These are a cheap and easy way to extend LLVM
     to support target-specific or other useful features without mangling
     the LLVM language. If you are interested in this, check out:

     The new Function::getIntrinsicID() method and llvm/Intrinsics.h file
     defines a simple interface to identify if a function is calling an
     intrinsic function and what it is. No LLVM passes should need to be
     modified ever to support intrinsic functions (assuming they treat
     external functions calls in a conservative fashion).

  2. Variable argument handling support. It is now possible to write
     printf in LLVM! This is the last big feature that LLVM has been
     missing that has prevented it from compiling "any old" C program.

     Specifically, support has been added for the C <stdarg.h> header.
     This involved the addition of three new intrinsics and a new LLVM
     va_arg instruction. These are documented here:

     Support for variable argument functions has been added to LLI, the C
     backend, and the X86 JIT. The _new_ C front-end also generates
     these correctly. A testcase has been checked in here:

Let me know if you have any questions or concerns.