Added popcount() and count_{leading,trailing}_zeros() builtins.

These map to the corresponding LLVM IR intrinsics.

Also fixed a comment in the makefile.

Two questions/issues:

1. I haven't updated the MSVC project/solution files to wire up the tests for this stuff; lacking a windows machine, I can try to do this in emacs or ... ?

2. The msvc/initmod.cpp file has changes when I build Halide; my assumption is that those shouldn't be checked in.  (Actually, is it an error that that file is in the git repository at al?)