This provides fixes for all ARCH_SIXTYFOUR's

...without actually testing them...

The two options are either build core_kernel (and related libraries) on
64 bit platforms twice - once natively and a second time for
js_of_ocaml (note; bytecode only needed).

This is the alternative approach which detects and adapts to the
platform at runtime.  All other libraries would need similar fixes.

I am sure Jane St. will have opinions about runtime cost /
maintainability of these different approaches.