Add stand-in arch for 32-bit initial module construction.

Silences a warning when compiling Halide on ARM about stdcall.
Add indentation to README.md for code snippets
More code indentation tweaks in README.md
Don't generate vst instructions with unsupported types.

Also, the old code didn't clean up the lets or fall through
to the strided store optimization if it bailed out.
Merge branch 'master' of github.com:halide/Halide
Reverted from MCJIT to JIT on Windows
Merge branch 'master' of https://github.com/Halide/halide
Merge branch 'master' into simplify-fuzz-testing