Added a new type GotoIfNotNode:

1. Created new data type in jltypes.c (and added it lower in the file)
2. Added to builtins.c
3. Added to boot.jl comments
4. Declared jl_gotoifnotnode_type in alloc.c
5. Exported jl_gotoifnotnode_type in julia.h
Use the GotoIfNotNode type.

* Added a case to construct GotoIfNotNodes in ast.c
* Constructor is based on surrounding code

Since I didn't edit any Julia code to handle the new type,
it is not surprising that it doesn't work.
It fails on line 38 of sysimg.jl, as it loads int.jl (at line 261):

~~~
error during bootstrap: LoadError(at "sysimg.jl" line 38: LoadError(at "int.jl" line 261: BoundsError()))
~~~
modified codegen to maybe generate code for gotoifnots