No varargs

This removes all use of varargs from the halide_runtime, with the exception of calls to android and ios logging functions. This solves nacl linking issues and some windows mess (e.g. we couldn't print floats properly on windows).

Ready for comments.
Did you intend to add printing of the code itself here? Note that the old code only printed the pointer of the code and the size, not the code itself. When I made that change, I thought it made more sense to dump the code from the compiler/non-runtime (when HL_DEBUG_CODEGEN > 0).