Update Kaleidoscope tutorial and improve Windows support

Many quoted code blocks were not in sync with the actual toy.cpp
files. Improve tutorial text slightly in several places.
Added some step descriptions crucial to avoid crashes (like
InitializeNativeTarget* calls).
Solve/workaround problems with Windows (JIT'ed method not found, using
custom and standard library functions from host process).

Patch by: Moritz Kroll <moritz.kroll@gmx.de>

Differential Revision: https://reviews.llvm.org/D29864

llvm-svn: 294870