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.

run_tests.py

@@ -37,10 +37,10 @@ parser.add_option("-g", "--generics-include", dest="include_file", help="Filenam
 parser.add_option("-f", "--ispc-flags", dest="ispc_flags", help="Additional flags for ispc (-g, -O1, ...)",
                   default="")
 parser.add_option('-t', '--target', dest='target',
-                  help='Set compilation target (sse2, sse2-x2, sse4, sse4-x2, avx, avx-x2, generic-4, generic-8, generic-16, generic-32)',
+                  help='Set compilation target (neon, sse2, sse2-x2, sse4, sse4-x2, avx, avx-x2, generic-4, generic-8, generic-16, generic-32)',
                   default="sse4")
 parser.add_option('-a', '--arch', dest='arch',
-                  help='Set architecture (x86, x86-64)',
+                  help='Set architecture (arm, x86, x86-64)',
                   default="x86-64")
 parser.add_option("-c", "--compiler", dest="compiler_exe", help="Compiler binary to use to run tests",
                   default=None)
@@ -58,6 +58,9 @@ parser.add_option('--time', dest='time', help='Enable time output',
 
 (options, args) = parser.parse_args()
 
+if options.target == 'neon':
+    options.arch = 'arm'
+
 # use relative path to not depend on host directory, which may possibly
 # have white spaces and unicode characters.
 if not is_windows:
@@ -345,10 +348,13 @@ def run_test(testname):
                     obj_name = "%s.o" % testname
                 exe_name = "%s.run" % testname
 
-                if options.arch == 'x86':
-                    gcc_arch = '-m32'
+                if options.arch == 'arm':
+                     gcc_arch = '--with-fpu=hardfp -marm -mfpu=neon -mfloat-abi=hard'
                 else:
-                    gcc_arch = '-m64'
+                    if options.arch == 'x86':
+                        gcc_arch = '-m32'
+                    else:
+                        gcc_arch = '-m64'
 
                 gcc_isa=""
                 if options.target == 'generic-4':