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.

examples/mandelbrot/Makefile

@@ -2,6 +2,7 @@
 EXAMPLE=mandelbrot
 CPP_SRC=mandelbrot.cpp mandelbrot_serial.cpp
 ISPC_SRC=mandelbrot.ispc
-ISPC_TARGETS=sse2,sse4-x2,avx-x2
+ISPC_IA_TARGETS=sse2,sse4-x2,avx-x2
+ISPC_ARM_TARGETS=neon
 
 include ../common.mk