Add support for ARM NEON targets.

Initial support for ARM NEON on Cortex-A9 and A15 CPUs.  All but ~10 tests
pass, and all examples compile and run correctly.  Most of the examples
show a ~2x speedup on a single A15 core versus scalar code.

Current open issues/TODOs
- Code quality looks decent, but hasn't been carefully examined.  Known
  issues/opportunities for improvement include:
  - fp32 vector divide is done as a series of scalar divides rather than
    a vector divide (which I believe exists, but I may be mistaken.)
    This is particularly harmful to examples/rt, which only runs ~1.5x
    faster with ispc, likely due to long chains of scalar divides.
  - The compiler isn't generating a vmin.f32 for e.g. the final scalar
    min in reduce_min(); instead it's generating a compare and then a
    select instruction (and similarly elsewhere).
  - There are some additional FIXMEs in builtins/target-neon.ll that
    include both a few pieces of missing functionality (e.g. rounding
    doubles) as well as places that deserve attention for possible
    code quality improvements.

- Currently only the "cortex-a9" and "cortex-15" CPU targets are
  supported; LLVM supports many other ARM CPUs and ispc should provide
  access to all of the ones that have NEON support (and aren't too
  obscure.)

- ~5 of the reduce-* tests hit an assertion inside LLVM (unfortunately
   only when the compiler runs on an ARM host, though).

- The Windows build hasn't been tested (though I've tried to update
  ispc.vcxproj appropriately).  It may just work, but will more likely
  have various small issues.)

- Anything related to 64-bit ARM has seen no attention.

main.cpp

@@ -243,12 +243,24 @@ int main(int Argc, char *Argv[]) {
     llvm::sys::AddSignalHandler(lSignal, NULL);
 
     // initialize available LLVM targets
+#ifndef __arm__
+    // FIXME: LLVM build on ARM doesn't build the x86 targets by default.
+    // It's not clear that anyone's going to want to generate x86 from an
+    // ARM host, though...
     LLVMInitializeX86TargetInfo();
     LLVMInitializeX86Target();
     LLVMInitializeX86AsmPrinter();
     LLVMInitializeX86AsmParser();
     LLVMInitializeX86Disassembler();
     LLVMInitializeX86TargetMC();
+#endif // !__ARM__
+    // Generating ARM from x86 is more likely to be useful, though.
+    LLVMInitializeARMTargetInfo();
+    LLVMInitializeARMTarget();
+    LLVMInitializeARMAsmPrinter();
+    LLVMInitializeARMAsmParser();
+    LLVMInitializeARMDisassembler();
+    LLVMInitializeARMTargetMC();
 
     char *file = NULL;
     const char *headerFileName = NULL;